Building a benchrest rifle

[Lee Martin]

Step 32 – Fitting the Main Spring
_________________________________________

We start with a Blitzschnell spring for the short action Remington 700:



Measuring 4 1/2", it's around 28 pounds in a stock bolt. Our action has a 2 1/8” space between the cocking shroud and the guide collar:



That’s a bit less than a Remington so clearly if installed as is we’d be over 28 lbs. To test this, we took an old Ruger cylinder blank and countersunk the face:



Using an electronic scale, a spring guide, the base, and a drill press we measured the pounds of force needed to compress it to 2.125”. The full assembly is placed on the scale and is tarred:



The press is brought down until the spring height equals 2.125”. Our scale does the rest. The uncut spring yielded 34 lbs. A few coils were cut and we tried again. Two passes got us to 28 lbs:



Measuring the compressed spring:



My goal was 23 to 24 pounds and we eventually got there:



I ordered two springs and will do this again to create a spare.

Calculating lock-time is up next...

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 33 – Calculating Lock-Time
______________________________________

Compared to other forms of shooting, lock-time (LT) isn’t as paramount in benchrest. Think about off-hand for a moment. There, high duration increases the risk of movement and by lowering LT accuracy usually improves. BR works off of fixed tops and rigid bags though. Firm bases and the free-recoil style so many embrace makes it less important on actions like ours. Even so, it can’t be ignored completely.

Lock-time is an idea thrown around a lot on the internet. Great discussion, but I often wonder if folks understand it in its entirety. For example, one guy argued trigger pull is the main driver. Another claimed heavy revolver hammers cause faster fall. Incorrect as that may be, there’s a lot to consider when figuring lock-time. Enough so that I thought it would be good to share the full computation. This piece focuses on bolt actions but in time I’ll do a revolver derivation.

By definition, lock-time is the interval between sear release and primer strike. Trigger travel and internal movement are other elements I’ll discuss later.

Let’s start with the main measurements:



FL = total length of the uncompressed mainspring (3.750)
L0 = length of the compressed and cocked spring (2.125)
L1 = spring length in the fired position (2.375)
X0 = spring compression when cocked (1.625)
X1 = spring compression when fired (1.375)
FPT = firing pin travel (0.250)
Fpt = firing pin travel to the primer (0.195)

We’ll also need the mainspring’s rate (K) in ft/lbs and the mass of the moving parts (M) in lbs-sec2/ft.

LT = [(cos^-1(L1/L0))/Sqrt(K)] x Sqrt(M)

Insolating each input:

X0 = FL – L0……3.750 – 2.125 = 1.625
X1 = X0 – FPT……1.625 – 0.250 = 1.625 – 0.250 = 1.375

The mainspring rate is less straightforward. To calc it these are a must:

D = the wire diameter
OD = outside diameter of the spring
R = mean radius of the helix
N = active number of coils

Our spring is 0.056” in wire diameter and 0.409 OD. It also contains 23 active coils. The radius is figured as follows:

R =1/2(OD – d) = 1/2(0.409 – 0.056) = 0.1765

Knowing steel’s modulus of elasticity, K is:

K = (D^4 x 11,500,000) / (64 x R^3 x N) = (0.056^4 x 11,500,000)/(64 x 0.1765^3 x 23) = 13.97 lbs/in

Converting to lbs/ft = 167.68 lbs/ft

The weight of the firing pin, cocking piece, and spring is 0.112 lbs. Dividing by earth’s gravitational constant yields mass:

M = (0.112 lbs) / (32 ft/sec2) = 0.0035 lb-sec2/ft

We can now calculate lock-time:

LT = [(cos^-1(2.375/2.175)/Sqrt(167.68)] x Sqrt(0.0035) = 0.00253 or 2.5 milliseconds (ms)

Is 2.5 ms good or bad? Well, 2.0 is deemed fast and aftermarket kits like David Tubb’s Speedlock approach that number; achieving less is tough. Old military rifles like the 1903 Springfield and Mauser 98 had from here to eternity LT’s, usually 6.0 to 9.0 ms. Most modern bolt actions land between 3.0 and 4.0 with their in-line springs. Non-linear ignition systems have much higher durations, 30 ms to over 100 ms in the case of revolvers. Ever wonder why single-actions can be harder to master than double-actions? Factory Ruger Blackhawks drop in 75 ms whereas Smith & Wessons are around 40 ms. That’s a sizeable difference.

Does 2 ms saved in a bolt action matter? For off-hand it does, but is anything gained on a benchrest gun? Human reaction spans 150 – 300 ms, many orders higher than my firing assembly. You’d think even a 10 ms lock-time would be nullified by the shooter. Not so. Remember, human reaction measures the interval between the conscious decision to act and the act itself. In shooting, that’s the time from deciding to pull the trigger to your finger being depressed. Once the sear releases, the ensuing mechanics are out of your control (though you can move the stock inadvertently thus effecting the shot). But assume you don’t influence the gun as is the case with free-recoil benchrest. In that scenario you simply touch the trigger and no other part of the gun. A chain of events takes place in the rifle and culminates with the bullet exiting the barrel. Throughout lock-time, vibrations and faint muzzle movements occur. They cannot be entirely eliminated. So lowering the void between sear release and primer strike reduces movement opportunity. Again, a millisecond here and there doesn’t payoff in benchrest like it does in other shooting forms. It is valid though when trying to eke out a hundredth or two in precision.

To summarize, lock-time can be shaved by:

1) Lightening the mass of the pin assembly
2) Increasing the mainspring rate
3) Decreasing the total travel

It's also good to understand the impact energy on the primer:

E = [(F0 + F)/2] x fpt

F0 = initial mainspring load = (K) x (X0) = 13.97 x 1.625 = 22.77 lbs
X’ = mainspring compression at fpt = X0 – fpt = 1.625 – 0.195 = 1.430 in
F1 = final mainspring load = (K) x (X’) = 13.97 x 1.430 = 19.98 lbs

Notice how the calculated initial load of 22.77 is very close to our scaled figure of 23?

E = [(22.77 + 19.98)/2] x 0.195 = 4.2 inch pounds

A stock Remington puts 5.0 to 5.5 inch-pounds of impact energy on the primer. Lab tests have found most rifle primers, to include thicker cupped BRs, only need 2.5 inch-pounds to ignite consistently. This is relevant for two reasons:

1) Our action has a lower lock-time than factory 700’s (it’s on par with the Tubb’s Speedlock)
2) Our impact energy is 25% less but still well above the minimum.

Lower lock-time reduces the window for vibrational interference. Lower impact energy also cuts vibrations across the receiver and barrel post-strike. The formulas show we achieved the best of both worlds – short lock duration without excessive primer hit. So how did we accomplish that? While our spring load is less than a Remington, we also cut the pin travel (0.250” vs. 0.270” on the 700). That shorter distance, while seemingly small, permits us to use less spring. Namely 5 lbs lower on the compressed load. A lock-time of 2.5 ms is fast enough, there’s no call for additional force. The reduction in LT would be immaterial and the uptick in strike energy would be unneeded. Harder impact also means bigger vibration and that’s a group killer. It’s no wonder the BR crowd avoids overly heavy springs.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[seancass]

Lee, you mention reaction time in the trigger squeeze and the decision to pull the trigger. I may be getting ahead of you here, but I'm curious about how you actually fire the gun. I'm used to the "squeeze squeeze squeeze BOOM" method, where there is no reaction time, you never make a decision to yank the trigger. Is it different in BR, no need to squeeze, just press when everything is how you want it?

You can defer this question to later, it just popped into my head as i read.

[Lee Martin]

A good question and one I'll cover later in depth. BR triggers are a bit more refined than traditional models. At 1.5 to 2.0 ounces, squeeze doesn't describe their let-off....touch does. Longtime benchresters are good enough to feel the trigger as it's depressed. But even then, when you touch them, they drop. In free recoil shooting, the tip of your finger is the only thing that engages the gun.

Recently a friend of mine tried one of our 1.5 oz Jewells. He couldn't believe its sensitivity....and he's an accomplished shooter. I'll get into the mechanics next week (Jewell's, Kelblys, and Farleys will be featured).

Thanks for inquiring. You're headed in the same direction I am with this.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[bradshaw]

Lee Martin's discussion of FIRING PIN FALL and LOCK TIME once again exposes why I consider Lee's bench rest articles vital reading for the revolver shooter. The difference noted between Ruger Blackhawk/SBH 70ms and S&W N-frame 40ms LOCKTIME is huge. A discussion of locktime and hammer fall characteristics between various revolvers must unite mechanics with the individual shooter. The most critical yet challenging co-ordination of FREESTYLE marksmanship is FOLLOW THROUGH. Therein lies the need for cool nerves, with BREATHING the radiator by which nerves are cooled.

Some persons just shoot a double action revolver better than a single action. Apples-to-apples, we're talking single action fire. For some, the long arc and tall hammer of a Peacemaker-type single action unnerves a cool squeeze. A hard Kansas wind scoured the range two of the four days at the 1978 IHMSA Internationals, full value left-to-right, 25-35 mph. So straight so full value so unwavering, this wind barely toppled at target. Panic swept the firing line as surely as the wind. I was among a core of silhouetters campaigning hard for creation of a Revolver Championship separate from Production Single Shot. "Keeping handgunning real," some said. Top revolvers included the 7-1/2" Super Blackhawk and S&W 8-3/8" M-29 .44's, with a smattering of S&W 8-3/8" M-27 .357's and Colt Pythons. A top shooter from Pennsylvania, Edna Huff, had her M-29 8-3/8" and, as I would take the line with my 8-3/8" M-29, we compared notes. Edna said she just did not shoot the Ruger well, while she handled the M-29 very well. Said she couldn't handle the Peacemaker grip. I suspect that the combination of Peacemaker grip with longer hammer fall worked against her.

We watched as revolver shooter who failed to dope the wind knocked over rams to right of the one they aimed at, sometimes striking two rams right. Revolver bullets drifted 4 to 7 feet at 200 meters. This is where you decide to drill dead tight on the pig @ 100 meters; the pig becomes the roadmap of wind drift, which adjustment is multiplied at 150 and 200 meters. Turns out I took high revolver, with Edna Huff right behind.

Lee mentions superior linear speed of the bolt action's firing pin, versus the angled spring and arc of a revolver hammer. Dick Casull told me that the flat spring of his Model 83 produced shorter lock time than Ruger's coil mainspring. I cannot vouch for that, although FA locktime feels fast. The old S&W drop forged mainspring seems no faster that the Dan Wesson coil spring. The Colt Python with light, short-radius hammer driven by a V-spring sports a fast locktime.

I appreciate Lee's formulaic math----despite my inability to follow it----as it measures the action of components in time.

Some years ago Remington introduced electronic ignition. About doubled the cost of a Model 700. Apparently the cyber kids were still in diapers, for in spite of applause in the press, sales lacked luster. Before that, rifle silhouetters tried an electronic trigger----was it called the "Green," made in Colorado?----which also failed to take over the firing line. Elgin Gates fit an electronic trigger to one of his custom Remington XP-100's. I remember it as less than perfectly reliable. The electronic trigger releases the standard firing pin, as distinct from electronic ignition, which requires a dedicated primer.
David Bradshaw

[bradshaw]

Lee.... speaking of vibration, do you know where sound waves go in a rifle barrel? Sound travels roughly 1,100 fps in air. Touch off a rifle, sound wave travels 16,000 fps, sound waves from firing pin and discharge bouncing off the muzzle 14 times as fast as the bullet reaches the crown, Add ignition & combustion, and it figures bounces back-and-forth the length of the barrel many times before the bullet escapes the muzzle. Is there a barrel contour more inert than others?

For those about to wrap their favorite barrel in lead, the velocity of sound in lead is 4,000 fps.
David Bradshaw

[Lee Martin]

David - I'll dive heavily into barrel harmonics later. Rigidity and deflection will be shown using....you guessed it, more math. I’ll also cover node tuning and vibration dampening. Some benchresters have in fact investigated sound influences with high tech equipment. Those trials went beyond the usual strain gauges at the chamber and muzzle for timing and solid state accelerometers at the muzzle (not that those are common). And you’re correct, sound travels much faster than the bullet. By the time the slug exits, sound has cycled 4 - 5 times end-to-end. In this capacity, the barrel acts like a tuning fork.

The goal in studying all this is to find preferred nodes, or the most repeatable barrel harmonic. Aside from the contour, bedding, and action, the variable we tune with is the load itself. Now barrels deflect mostly in a vertical plane. What we want is bullet exit at the top or bottom of that deflection (graphically the vibration node looks like a sine wave). Those node peaks and valleys are where the barrel is vibrating, or moving, the least.

I’ll end with this before I jump too far ahead. In short-range benchrest, the PPC is loaded to where the bullet leaves the barrel in roughly 0.002 seconds. LV and HV contours vibrate for 0.120 - 0.150 seconds in their entirety. That vibration isn’t constant and spikes and then declines rapidly. The window we’re trying to manage is extremely narrow. More to come on all this...

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 34 – The Waiting Game
____________________________________

Without the stock, we’re at a stopping point on our action. The trigger set, rear contour, and overall length have to be matched to the glass.

We’ve used a fair number of benchrest stocks over the years, principally McMillans and Harts. For this build I narrowed it down to three:

McMillan’s HV Edge



Kelbly’s Klub



Tom Meredith’s TMBR



There’s a lot to think through when selecting one. The top elements are tracking, weight, and legal for competition. Composition is another factor, one that transcends making weight. Most BR stocks are time proven fiber-glass with polyester resin. Blends of fiber-glass and carbon fiber are also common. Still others employ pure carbon-fiber with epoxy resins to rigidly bind the matrix. I’ve used all three and they’re stellar. The choice of materials is really driven by comp weight constraints. Light varmints in particular present a challenge with the 10.5 pound cap. Fortunately some manufacturers offer class specific models. One such company is McMillan. Their Edge LV wears a graphite shell, thereby lowering weight 1/3 (average = 22 - 24 oz). Their HV Edge is done with the traditional fiberglass cloth and scales around 38 oz.

Tracking – whether you practice free recoil or not, the stock should fit the shooter. I recommend holding a few before dropping the cash (btw – they range from $400 to over a grand). Secondly, it must track well on the bags. Tracking describes the gun’s movement shot after shot. I won’t get into the specifics here, that’ll come later. But rearward slide and repositioning is huge in precision shooting. It has to be fluid, in-line, and repeatable.

A beautiful composite-cocobolo BR by Roy Hunter. They list for $1,100 - $1,200:



The International Benchrest Shooters (IBS) association and National Benchrest Shooters Association (NBRSA) have similar rules around stock legality. To quote from each rulebook:
__________________________________

IBS Heavy Varmint (same applies to Light Varmint but total weight is reduced to 10.5 pounds):

Any rifle having a safe manually operated firing mechanism, weighing not more than 13 1/2 pounds inclusive of sight, with a stock having a flat or convex forearm and total stock width not more than 3", having a toe formed by a straight line drawn from the toe of a substantially vertical butt with a 13 1/4
inch pull starting at a point at least 4 inches below the axis of the bore and extending forward to the centerline of the barrel at a point no more than 18 inches forward of the bolt face a barrel not less than 18 inches long, a diameter at and not more than 5 inches ahead of the bolt face of not more than 1.250
inches and a diameter at any point between the muzzle and 5 inches in front of the bolt face not greater than would be defined by a straight taper between such point 5 inches in front of the bolt face at 1.250 inches diameter and the muzzle at 0.9 inch diameter at 29 inches.
___________________________________

NBRSA Heavy Varmint (same for LV except 10.5 lbs on weight)

A Heavy Varmint Rifle is any rifle having a safe manually and mechanically operated firing mechanism and must not weigh more than 13 1/2 pounds, inclusive of sights. The stock should have a flat or convex forearm not more than 3 inches wide and having a toe formed by an acute angle that conforms to the Varmint Rifle diagram (on page 68 of Rev. Ed. No. 36). The barrel would not be less than 18” long forward of the bolt face and a diameter of not more than 1.250 inches from the bolt face forward 5 inches. From said 5-inch point, the diameter would not be greater than would be defined by a straight taper between such point and a muzzle diameter of 0.900 at 29 inches.
_____________________________________

The rules are easier to grasp through this illustration:



I picked McMillan’s HV Edge after talking to a few competitors that have used them. They all cited:

1) Oustanding rigidity
2) World class tracking
3) Ease of planting on the rest (due to the Edge's lower center of gravity and abrupt sidewalls)

McMillan HV Edge – note the exposed buttstock and built-in weight dampening.





I ordered mine last September after hearing about McMillan's backlog. They quoted 4 months, then pushed delivery to March-April. Hopefully it’ll be in-hand soon. I want to acclimate to the gun this summer and compete in some fall matches.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

The Wait Is Over
_____________________________________________

After 6 1/2 months the stock finally arrived:



Basic 1.20” inlet:



Weight dampening system:



A very squared, but still IBS & NBRSA legal, fore-end:



McMillan will paint these but I asked for bonded glass. I have a specific color in mind and want to shoot it myself.

It weighs 35 ounces which is in the expected range. With this in hand we’ll start laying out the action this week.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[Lee Martin]

Step 35 – Laying Out the Action Contour
_____________________________________________

I was supposed to shoot benchrest today, but it’s raining. So we decided to use the time to work on our receiver.

To correctly position it in the stock, we’ll account for the trigger set and tang inlet. It sounds easy but you have to be careful here. Ultimately it comes down to how you’ll affix the barreled action. In benchrest, the two main methodologies are pillar bedding or straight glue-in. We’ve done both, having achieved competitive precision from each. Let me start with glue-ins, a process that emerged in the late 1970’s.

Gluing an action to the stock involves set-screws and bedding compound. The trigger is still attached with two screws and some guys add a third anchor. The action and a few inches of the barrel are then mated with bedding glass or gels (Acraglass, Devcon, MatchGrade, etc). Araldite, a structural epoxy, is also recommended. The difference between this and traditional bedding is you don’t use a release agent. Upsides include consistent heat transfer, uniform purchase, and high rigidity. The downside is the gun doesn’t come apart as easily and in benchrest you’re constantly swapping barrels. But there are a few tricks to breaking the bond. The old-schoolers put an iron on the action and barrel around the chamber. This usually loosens the compound enough to pull it. Hot air guns can also soften the upper and rear portions of the glue. Too much heat will degrade the stock though so you have to know what you’re doing.

My light varmint .222 done as a glue-in. You’ll notice a safety bolt just in front of the trigger guard.





For ease of disassembly I prefer pillar bedding. On this rifle we’ll use three and that process will be covered at length. But in laying out the action, we have to be cognoscente of where they’ll sit.

We machine our own pillar system. Examples of some commercial makes:

McMillan pillars



Score-High pillars



Step one is to compare my uncut receiver to one of our completed actions:



We landed on a length of 8.5”. Cutting the rear in a band-saw:



The end is then faced in a lathe:







A surface plate is leveled and the action is put in v-blocks. You’ll see the tail is coated with layout paint:





Step one is to re-check the O.D. It still measures 1.454”.



The top of the action in the blocks is 3.060” off the deck. Now to determine the mid-point of the receiver:

Mid-line = (1.454” / 2) = 0.727”
Height off the table for the mid-line scribe = 3.060” – 0.727” = 2.333”

You don’t want the bolt to stop at the mid-point. It needs to go further to seat the lugs. Determining the handle notch:

We begin by dividing the bolt handle by two:

0.375” / 2 = 0.187”
Top-line of the notch from the deck = 2.333” + 0.187” = 2.520”
Bottom of the notch = 2.333” – 0.187” = 2.146”

Setting the scribe and lining the layout paint:





Top and mid-line indicated:



All three lined:



The same calculation is used to locate the top recess line:



The bolt is locked and firmly set against the raceway. A rod is then used to measure the distance to the back of the cocking shroud:





The bolt cam is drawn and includes a 45 degree transition:







There are no calculators or computers in our shop. Dad is unbendable and still does everything long-hand:



Next-up is a lot of milling.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[paulg]

As Frank Barone would say "holy crap"! Chasing a bullet one formula at a time, very cool!

[Lee Martin]

Step 36 – The Bolt Inlet
_________________________________________

The action is secured in a Bridgeport and a large end-mill is chucked.



We removed the top portion first. Each pass was 0.030” deep:





Care has to be taken as we approach the cam. Cutting too far in can hose bolt timing:



It took ten passes to transverse the sidewall:



The table was moved in and we started on the port side:



We’re cutting to the top index line:





The roughed upper and lower contours:



A 3/8” bit makes the bolt drop in 0.020” increments.







A smaller end-mill does the finishing passes:



It may surprise you, but after these steps the bolt no longer fits the action. By milling the upper right quadrant, the inside diameter contracts a thousandth or two. This dimension was easily corrected with our hone:





-Lee
www.singleactions.com
"Building carpal tunnnel one round at a time"

[Lee Martin]

Step 37 - Contouring the Tang
______________________________________

Using a completed action we set the tang slope at 24 degrees off the centerline axis.



The table angle is positioned and a scribe pass is made:



A 3/4” end-mill is slowly fed in:



With the larger bit the Bridgeport speed was dropped from 210 to 125 FPM. Our steel is very hard so we kept cuts to 0.030”:





Here’s the tang in final form (polishing still required):







We again Rockwell tested the action. Hardness was around 31 so we’ll be heat-treating it after the trigger is set, the top is tapped, the bolt stop is installed, and the pillar locations are drilled.



-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"

[squawberryman]

Suffice it to say there's no way anyone (retailer) puts the thought into building an action that the Martin team does. But I gotta ask, do any of those actions that you've previously specified by a comparison of accuracy, rigidity, etc. come near this? I would have to think that if a non machinist were a world class shooter the answer would be yes, he bought a great action. To think these high a quality items are made and sold as just an "item" is baffling to me.

[Lee Martin]

I appreciate that Greg. In all honesty, we're just a couple of guys with a basement machine shop. No deadlines, no customer reviews, and no budget allows us to build these on our terms.

Now comp actions absolutely get this level of attention. Examples include Bruce Thom's BAT, George Kelbly's Panda, the Stillers, and Jim Borden's Rimrock. Commercial makes like Remingtons, Sakos, and Savages are good but don't come close to this type of precision. Nor should they.

I'll equate Benchrest to another passion of mine, namely drag racing. It isn't hard to build a 10 second ride. It's when you try to run sub-8's that the engineering gets put on steroids. There are a lot of actions that when mated to a good barrel and proper tune can print 1/2 MOA or less. I'm sure they'll even dip into the 2's on occasion. But benchrest is all about repeatability. If your gun won't do 1/5" or less all the time in still conditions, don't waste the components. The game is that tight.

The TLC I've illustrated is a requisite, but no guarantee of results. We'll bolt all this together and hope for the best. Of course the element I haven't touched on yet is the shooter. This action is being built to put me in a position to win. If I can't manage the bench, the loads, and the environment it's all for naught. And that's what seperates a novice like me from the Boyers, Gonzales, and Geracis of the world.

-Lee
www.singleactions.com
"Building carpal tunnel one round at a time"