bullet weight v. bullet profile

[c.r.]

I'm not sure exactly how to phrase this question, so I'm going to need y'all to read a little between the lines. So please allow some wiggle room on the details. If my numbers don't work, just substitute numbers that do.

Part One
lets say we have two bullets cast from the same mold (lets say Lyman #429421). The bullets will have the exact same profile and ideally the same ratio of weight in the base to the weight in the nose.

The only difference is the alloy used. This difference results in: Bullet #1 comes out at 250 gr. Bullet #2 comes out at 240 grains. Once the bullet is assembled, they will have the same OAL, the same case capacity, etc.

Part Two
Now we have two bullets cast from two different molds. However the same alloy is used and somehow the molds produce two bullets both weighing 250 grains.

The difference here is that bullet A and bullet B are different profiles. Bullet A has more of the bullet below the cannelure. This results in reducing the volume of the case when compared to Bullet B being used.

Question
of course it's going to matter how much the capacity of the case is reduced, but which of the above "parts" is likely to have the largest effect on pressure?

Since 240 is ~4% less than 250, let's say the case capacity is reduced by ~4% in part two. I'm not sure if that's a reasonable assumption, but hopefully y'all can understand where I'm going with this question.

thank you,
c.r.

[jefats]

OK, I'll make a stab......

Part 1: Assuming everything else is equal, it would seem to me the heavier bullet of the two from the same mold will produce the most resistance, ergo more pressure than the lighter one.

Part 2: I believe the bullet which has more of its surface contacting the lands will produce greater pressure. Naturally, if it's the one with more stuck down in the case it will be more noticable. Swapped around it may be a wash.

[Aggie01]

I may be completely wrong here, but I'm pretty sure that Bullet A in part two will increase pressure the most.

The two in part one should not impact pressure, the biggest impact from that will come from how the two allloys perform at the same pressure.

the bullet with more length below the cannelure will increase pressure in two (maybe three) ways:
1: reduced case capacity (you know this one)
2: increased neck tension from more bullet in the brass
maybe 3: you can get really nitpicky and consider actual bearing surface as marginally increasing friction. this should really not be an issue, since any increase in inertial moment due to frictionwould not be discernible from my #2 note above, and once a bullet is moving it doenst take much to keep it moving. Bullets with long bearing surfaces tend to be more consistent as well.

let me know if I figured out where you are going, of if i led you off the path.

[Frank V]

C.R. If you use a good loading manual & start below max you should be able to work up similar load for either bullet. It may not be the same powder charge, but results should be similar & you should be able to use the loads more or less as the same load. Hope we haven't confused you. The use of a good loading manual is paramount. Thanks Frank

[nobearsyet]

I agree with Frank

[c.r.]

both of y'all are right on track with my question and actually brought up some issues that I should have included in my question as well.

However there is one statement that I not quite clear on. "The two in part one should not impact pressure, the biggest impact from that will come from how the two allloys perform at the same pressure."

I guess they way i saw it was a 270gr. keith bullet would generate higher pressure than a 250 grain keith bullet when all other variables are the same. However, after a little thought, I suppose that would mean that 270 and 250 bullets wouldn't be identical in dimensions..........similar, but not identical. It sounds like it is very unlikely to have "identical" bullets that have a difference in weight that is large enough to have a material effect.

[nobearsyet]

The 270 probably would increase pressure as it would take more to ge tiut oging

[c.r.]

here's what triggered this question. There was discussion on the previous forum about the FA mdl 97 in 45 Colt and even discussions about FAs in 475. The issue of OAL was addressed and some of the solutions were to "crimp over the front driving band". This got me to thinking as to which would have more effect on pressure.

I reckon I tried addressing that with my question in a backwards way

thanks,
c.r.

[Frank V]

C.R. Glad we helped. Reloading is a lot of fun when done safely & most of us get a real thrill out of developing a load for a specific gun, when it all comes together it's really satisfying, go slowly & I'm sure you will come up with loads that suit you & your gun. Frank

PS: If you crimp over the front driving band it will seat the bullet deeper in the case. I'd just reduce the load one grain ( assuming you're not at the top) & work up from there. FV

[nobearsyet]

I would drop a half grain of powder from the starting load when crimping to a shorter OAL tha nthe book states (as a safety measure, but the nagain I am the guy who shoots Keith Loads in a Charter Arms with no ill effects) and work up from there over the chrono, when I hit max velocity I'd stop, if it isn't showing any signs of pressure and that's still not enough for you, proceed wit hextreme caution, and not ver ymuch