pressure question for those with testing gear

[tazman]

Does having a cast boolit 3 thousandths or more oversize give a significantly higher pressure than one cast 1 thousandth over? If it becomes significant, at what oversize range does it matter?
Mostly concerned for pistol use but would also apply to rifle use.
My only concern about this is a situation where you are running near max loads and might run into a problem if it is significant.
I tried the search engine but came up with nothing.

[Tatume]

Oversized cast bullets will not run pressure up significantly, provided the load is reasonable to begin with.

[geargnasher]

Loose recollection here from some positive testing I've seen is that you don't see much difference until .003 or so is reached, then it's like 3K PSI per thousandth after that with typical alloys. Someone else with a better rembemberer will chime in hopefully.

Gear

[Outpost75]

As long as any increase in bullet diameter is not so much as to prevent the case neck from radially expanding to safely release the bullet, the effect of a small increase in bullet diameter, up to about 1%, does not have a significant effect upon shot-start engraving or extrusion force, or chamber pressure.

Case in point, in the 1980s, testing .303 British MkVII, Mk8z service and SAAMI Reference 180-grain softpoint, as well as proof ammunition in two radial-copper pressure test barrels, one having nominal ".30 caliber" bore and groove dimensions, four grooves 0.074 wide, bore .3000, groove .3080 with ten-inch twist of rifling, versus one chambered with the same reamer, having normal ".303 British" dimensions, five equal width lands and grooves, bore .3030", groove .313" with ten-inch twist of rifling, the average pressure rise was about 3500 cup for the tight barrel with the service loads and SAAMI reference softpoint ammunition and about 5500 cup with the proof ammunition.

These tests being with JACKETED bullets and factory loads. The pressure difference, while significant, does not exceed design limits for a sound, modern rifle.

I have two Long Branch No.4 rifles which were rebarreled with standard .30 caliber barrels, using a SAAMI-dimensioned .303 British pressure barrel reamer, which digest all manner of factory and arsenal .303 loads with no hard bolt lift, sticky extraction or other issues.

[tazman]

Thanks gentlemen. I was hoping this was the case, but I have learned not to take things for granted.
Better to ask than wear the slide or the bolt.

[Larry Gibson]

I have pressure tested that "theory" (the "you don't see much difference until .003 or so is reached, then it's like 3K PSI per thousandth after that with typical alloys") with both rifle cast bullets and handgun cast bullets and have not been able to find it correct. The time pressure curve is slightly different but the cast bullet, even with linotype alloy, is swaged to groove diameter with much less psi than the load develops. The swaging of the bullet is done very early in the time/pressure curve as it occurs within the 1st bearing length of bullet travel in the barrel. After that the larger diameter cast bullet is the same diameter as the slightly smaller cast bullet with the internal ballistics being the same for both there after for the remaining trip down the barrel. Any difference in initial psi falls well with in the shot to shot psi ES and is essentially immeasurable.

There can be a very measureable difference in oversize jacketed bullets though depending mostly on jacket material and construction. Pressures using such can easily get out of hand if just substituting oversize jacketed bullets in a given load. However, if a load is worked up using the oversized jacketed bullets then the pressure should remain within acceptable standards.

In this case, as with many others, what is with cast bullets isn't necessarily what is with jacketed bullets.

Larry Gibson

[tazman]

So basically the diameter of my cast boolit will have no effect on pressure as long as it will chamber in the firearm. Good to know I don't need to worry about that aspect.

[geargnasher]

It depends on the situation, I know someone at some point did a CB experiment using pressure test data (Idaho Sharpshooter perhaps?) and there WAS some slight increase in peak pressure. This is one of those things where the results depend on a lot of different factors, but unless you're pushing the ragged edge of pressure limits already, I doubt you need to worry about it.

Gear

[tazman]

I have no intention of pushing high pressures. I did that once right after I started reloading about 40 years ago. I learned my lesson then.
Good function and accuracy is all I look for.