how much difference does .001" make???

[SSN Vet]

O.K. let's just start by saying, I don't care if I save money loading 7.62x39

I've been working up loads for 124gr FMJ from Goldenwest w/ decent results....these bullets mic at .311

I've also been casting WDWW w/ the .312-155 Lee mold and have sized them to .311 as well as shot them as molded at .312 +/-. Still have a ways to go working up this load and plan on slugging my barrel with 12 ga. 00 buck pellots tonight. Based on my previous attempts to slug the barrel with my cast bullets, I have reason to believe that the barrel groove dia. is over .312 at the muzzle.

So.....

I bought a box of Hornady .312 dia. 155 gr. SP bullets the other day.....kinda spacing it....thinking .312 lead works, so .312 jacketed should be just fine ... forgeting of course that it takes more pressure to "cram" the jacketed bullet through the rifling.

So, my question is....should I

1. use normal 7.62x39 load data and not worry about it.
2. back off the published 7.62x39 loads to allow for possible higher pressure
3. push them through my Lee .311 sizer die with a smidge of Imperial wax.
4. buy a Lee Enfield
5. put them on the back shelf and forget about them.

and

How much does the standard groove diameter for a given calibre rifle run over the jacketed bullet diameter for it's assoicated cartridge?

I ask because when I get a reliable slug at both ends of the barrel, I'll have another data point to throw at the decision.

 

[Grump]

Here's a clue:

Lapua or one of those hyper-priced European joints has sold .309 match bullets for use in the .30 calibers for decades.

 

[rcmodel]

In 7.62x39, I would be more concerned, and cautious, about the 155 grain weight then I would be about .311" vis .312". If you have load data for that weight bullet, use the starting load and work up as usual.

Running them through a Lee sizing die will ruin them accuracy-wise because it will loosen the jackets on the cores.

The first thing to do is slug the bore, while paying special attention if they got loose towards the end of the muzzle. That will indicate cleaning rod wear, and there is nothing you can do about it with over-size bullets.

If the bore is smaller at the rear of the barrel, that's still going to be the size of the bullets when they get to the loose part near the muzzle.

rcmodel

 

[Jim Watson]

As good old Parker O. Ackley said, by the time the bullet travels its own length, it has been sized to fit the barrel. Excess pressure is caused more by tight necks that won't let the bullet get out of the chamber readily.

If your barrel is larger at the muzzle than the breech, you will be limited on accuracy no matter what size bullet you use.

 

[mkl]

It makes no difference as far a pressure is concerned.

Old wife's tale for the most part that a few thousandths of a inch in bullet diameter can make a difference..... And I'm talking about a few thousandths (say less than .010 or so) and modern rifle cartidges only.

Given the pressures we are talking about in a modern rifle cartridge, the bullet has about the same consistancy as if your were to shoot silly putty. In other words, forming the bullet to the barrel is only going to take pressures in the 10 to 15 thousand psi range, where we are developing pressures of 45 to 60 thousand psi.

Back in the "old days" P.O. Ackley, one of the greatest gunsmiths to ever live did some pressure testing in a fully instrumented pressure barrel. He was trying to find out if a "bigger" bullet made any difference in pressures. Long story short, he used a factory loaded 30-06 cartridge and measured the pressure (57,300 psi). Then he opened the throat of the barrel up so he could seat an 8mm bullet. He pulled the 30-36 factory bullet, expanded the neck and seated an 8mm bullet in its place. Now we are talking about shooting a .323 bullet down a .308 barrel.

Guess what? No increase in pressure.

See Page 76 in Volume II of P.O. Ackley's Handbook For Shooters And Reloaders for complete test details.

Long way of saying I think you will be safe with .001 oversize bullets

 

[Harpo]

Good stuff here! I was worried about using .454 instead of .452 lead bullets in my 45 Colt. Seems like it shouldn't matter a bit!

 

[Galil5.56]

Guess what? No increase in pressure.

Didn't the pressure actually measure less in that experiment??? Seems I recall it did. I always wondered how this test would have panned out using a Barnes x type, or similar bullet. I always think of what you wrote when people give me "that's B.S." look when I tell em I load some cast 9mm bullets as big as .359"... If it will chamber fine, it will shoot fine.

 

[machinisttx]

Just curious, but what rifle are you loading for? Ruger barrels are .308" IIRC, but other manufacturers may vary anywhere between .309" and .314".

I need to start loading for my free mini 30.

 

[SSN Vet]

I got what I believe is a reliable slug of the barrel tonight....

.312 accross two of the pairs of lands
.3125 accross the other pair.

I played around making a dummy round and the bullet won't feed if I crimp to the crimp groove, but their seems to be plenty of neck tension, so I'll just seat to the max. OAL and then be careful not to get a compressed charge.

Aside from measuring down to the powder and then keeping track how deep the bullet is going into the case and then doing the "shake-a-shake-a" test, is there any other way to check for a compressed charge?

 

[SSN Vet]

what rifle are you loading for?

A 16" Saiga sporter