Not Sure If I've Had An Epiphany

[JohnH]

or just another hair brained idea. Was thinking about why plain base boolits in rifles seem to be pretty much limited to less than 1600 fps or so. I know a few exceed that but for most us that seems to be the upper speed. In thinking about what the difference might be it struck me that the muzzle pressure is higher with the higher velocity one attains. Getting 1400-1500 with plain base and light charges of fast powder is pretty easy, but to exceed 1600 fps one needs to burn slower powders, which translate to heavier powder charges which translate to more gas volume and higher pressures further down the barrel. I begin to think that as muzzle pressure climbs, the ability of a plain base bullet to deal with the barrel restriction of the bases plasticity in the transition from barrel to air may cause tipping or other erratic changes in the bullets base. Am I nuts?

[longbow]

Nope! Not nuts.

Any imperfections in the boolit base can be a problem for accuracy and at higher pressures gas cutting tends to occur so as the boolit leaves the muzzle there could be gas leakage occurring as well as erosion of the edges of the base from gas cutting happening further back in the bore. Both will cause problems by tipping the boolit where there is leakage.

Try shooting into a media that will allow recovering your boolits and examine them. If you start with low pressure loads and work up I am betting you will see gas cutting show up at around 1600 FPS or so depending on caliber and boolit fit. I always try to recover boolits especially if a load is not accurate. I often see gas cutting. My .44 Mag Marlin is a prime candidate for this and the bottom driving bans is often "scalloped" from gas cutting even though boolits are 0.002" to 0.004" over groove diameter depending on mould and whether I size.

Another issue that can show up with fast twists is skidding. I found that with some loads in my .303 Lee Enfield with 1:10" twist that I was getting some skidding/swaging showing wider grooves than lands on the boolit (rifling lands and grooves in the barrel are same size). Oven heat treating the boolits solved that... at least for that load. I didn't push beyond it. Of course that affects GC boolits as well as PB boolits. Some say the gas check "keys" into the boolit to help rotate it but I do not buy into that. Too little contact area and no real "key" effect. The gas check is there to stop gas leakage at the base of the boolit. I am too lazy and cheap to use gas checks so mostly use of COW filler. Not as effective as a gas check but it works well for my loads.

So there is another test you can run ~ use PB boolits with no filler and see at what velocity accuracy fails then do the same with filler. I have never bothered to run a formal test but do find that filler helps in my loads. One other observation here is that GC boolits shot without CG seem to show worse accuracy and more gas cutting than a PB boolit. Also, GC boolits shot without gas checks but over COW filler see significant swaging of the GC shank by the filler. I have recovered boolits with GC shanks that looked more like a boat tail than a GC shank.

My thought and experiences anyway.

Longbow

[44man]

Most is correct Longbow. The GC helps stop skid so channels do not open but can be overcome too.
I shoot mostly PB and make my boolits hard and if expansion is needed I soften the nose but not the drive bands.
Nothing will tell more then recovered boolits.
I don't think additional damage is done at muzzle exit, damage is before then.

[Fishman]

I would think that the gascheck would rotate easily on the boolit base, given the pressures and rotational forces involved. If it rotated, it would protect from gas cutting even if the rest of the boolit skidded. This could be why nobody has every shown a consistent difference in effectiveness between the old Lyman press on checks and the crimped ones made by Hornady.

I can present no evidence to this, just a hypothesis.

[popper]

John - I shoot 30 cal PB (all but 0.020" of the GC shank removed) to pretty good fps with good (for me) accuracy. If the rifle crown is good the boolit base is the problem. I use a smooth sided boolit, it doesn't skid. I did a PC vs HiTek test. HiTek won as the PC on the base edge was not even. I got a nice 1" dia. ring pattern on the target @ 100. HiTek gave me some vertical stringing (me). IMHO you need the moderate speed powder to keep pressure on the base but fast enough so it all burns in the barrel.

[JohnH]

Thanks guys.

[BAGTIC]

I suspect that the bullet expansion due to setback will produce a more or less swage fit in either design of gas check.

[BAGTIC]

If a bullet starts off larger than the groove diameter it is going to be a swage fit so where does the initial gas cutting begin.?

[runfiverun]

along the feathered edges from the excess lead being pushed back from the swage down.
the gas check prevents a lot of this by providing a place in front of it for that excess lead to go, and by it being harder and checking the gas.

I'm pretty sure the gas check is also what lead many to think that a harder alloy is the way to go.