I slugged the bore of my Finnish M39 to find out it has a .310 bore. Will using a Hornady .312 bullet be cause for concern as far as increased pressure?

The barrel is mint and it headspaces fine. I'm not interested in max velocity but have no way to measure pressure except by examining the brass and opening the bolt.

Yes, pressure will be increased by using a bullet, especially a jacketed bullet, that is 0.002" over bore diameter.

No, if the pressure peak of swaging the bullet down to bore size is not concurrent with the pressure peak of the powder burning, then the absolute peak pressure does not change.

I have shot .323 bullets in a 303 Brit, 45 acp bullets in a .410 shotgun, and others.

For those who can't believe anything unless they read it in a load book:

P.O. Ackley 1966 "Handbook for Shooters
and Reloaders Vol 2" chapter 7
"additional pressure tests":
"..30 cal barrel pressure barrel was fitted to the test gun, but the neck and throat was enlarged to accept the 8mm bullet, with the bore remaining the standard 30 caliber. A Remington factory 30-06 cartridge with the 150 gr bullet had been tested and previously gave 57,300 psi, for a velocity of 3030 fps. The the bullets were pulled from two more Remingtom 150 grain cartridges and were replaced with 8mm 150 grain bullets. To everyone's surprise, although the velocity was rathererratic, these loads averaged 2901 fps, with a pressure of 40,700 psi."

Clark, all I have to say is I'm glad I don't shoot any of YOUR reloads!

Hi Nocked,
Just curious as to how you slugged your barrel. If you just pushed a lead slug down it even with a fair amount of force to start it,it's quite possible that you didn't get full expansion with the lead slug. You have to put something like a dowel that is bore sized down the muzzle end and expand the slug out to full dimension against it to get a full slug size and measurement. I would think there is more chance that the slug is not expaned enough plus it can be difficult to get a precise measurement on a rifled slug. Your talking .001 per side which isn't hard to loose.
Take care,
470 Mbogo

Clark has it right.

I also have a Finnish M39, which is outfitted with a strain gauge. The barrel slugs at .3102", which, I'm told, is about what practically all Finns slug. I routinely shoot Hornady bullets in it. These sometimes "mike" at .312", and sometimes at .311". With 50 grains of Varget behind one of these pills, and with CCI200 primers and a 2.880" COL, I get about 52 KPSI, which is within my personal comfort zone. Commercial S&B 180 grain measures 58 KPSI, which is too hot for my comfort.

As to the precision of the strain gauge system, you'll have to wait for my article in the July "Varmint Hunter", where I review the precision of the Shooting Chrony, the CUP method, the piezoelectric method, and the strain gauge method.

Denton,
I found the commercial S&B soft point 180 gr ammo to be very accurate. But when I tried to reload, the primer pockets had a big radius on the bottom and i had to ream the pockets. To make matters worse, the reloads were not accurate.

I gave up on them and use Lapua brass now.

Hi 470 Mbogo, I slugged the bore with an egg shaped fishing sinker a little larger than the bore and wooden dowel rod. I measured across the lenght and width of the slug.

Clark, Thanks for the info. I use Lapua brass because I had the same problem with the S&B. Lots of my primers were cratered with that stuff.

Fire a round. Now drop one of those bullets into the mouth of the case. Does it enter freely? Or is the fired case mouth too tight to admit the bullet??

If it drops in easily, and slides back out easily, you can fire them with no worries about increased pressure. Oversize bullets ARE ONLY DANGEROUS IF THEY JAM IN THE CASE AND WON'T COME OUT INTO THE BORE AS THEY SHOULD WHEN FIRED.

Clark is right, stans and vip are both WRONG!! In the late 50's or early 60's, a guy sent an Arisaka 6.5mm to the NRA that he had rechambered to .30/'06 WITHOUT ANY OTHER CHANGES!! He had been firing it with no problems!! The NRA also test-fired it, and were amazed at the "arrow-like" appearance of the .308" bullet that had been swaged down to 6.5mm as it went through the barrel.

Ackley also proved this is possible, (he didn't think an 8mm bullet would b;low up a .30 cal rifle) and offered a reward ($50,000??) to anyone who could prove to him that a rifle could be damaged with oversize bullets, PROVIDED THAT THE CASE MOUTH FREELY RELEASED THE BULLET ON FIRING!! No-one ever won this bet against him!!

[ 05-26-2003, 00:11: Message edited by: eldeguello ]

Hot Core..

I'm woefuly ignorant on the case expansion method. However, just a cursory look at the resolution of the system says that if .0005" corresponds to roughly 50 KPSI, and the mike can resolve .0001", then the resolution of the measurement system is around 10 KPSI. So that has not encouraged me to do a full measurement analysis on case expansion.

Maybe we ought to do a statistical work-up, and see what we get. Could be the basis of a good article.

eldeguello, I'm not wrong! I am glad I don't shoot reloads with severely oversized bullets. This rationale is what tends to cause injury. It might work most of the time but eventually the odds are going to get you. I'm not talking about swaging a thousanths or so all the way around but when you start bumping it up a caliber it's like playing Russian Roulette. Akley did alot of destructive testing and concluded that the Arisaka was almost impossible to destroy. 500grains posted of his trying to destroy an Arisaka. A few years ago I saw a young man bring his father's favorite 25-06 sako into the gunsmith. He was shooting it with his friend who happened to be shooting a .308. I think you know where this goes. Anyhow after the the gunsmith was able to hammer the action open what was left was a shattered stock, bulged magazine, and a bolt that was set back a few thousanth into the action. THE NECK RELEASED THE BULLET JUST FINE WITH PLENTY OF ROOM TO SPARE!! However, the case no longer looked like a 308 and the bullet swaged itself completely through the bore. You may want to rethink your statement "oversize bullets ARE ONLY DANGEROUS IF THEY JAM IN THE CASE AND WON'T COME OUT INTO THE BORE AS THEY SHOULD WHEN FIRED." I know this is shooting a different case and all but just wanted to show the chamber sealed and the bullet cleared the bore but the pressures were so high from swaging the bullet that the case head/primer failed and the gases exited through the magazine well. This was lucky for the shooter as he only sustained a splintered hand and an angry father.

If you are talking about "BORE" diameter, that is the diameter of the "LANDS" of the rifling. It is normal to use bullets "about" .008 inch in diameter LARGER than the BORE diameter. Such as .308 dia bullets in a .308 Win caliber with a .300 diameter BORE (which usually has a GROOVE diameter of .308). The bullets are "usually" the same diameter as the GROOVE diameter (not the BORE diameter) in most rifles. If I am mistaken, someone please correct me. Have fun, Safeshot

That was interesting.
If the peak pressure from burning in is when the bullet has traveled 1", and a .308 in a 25-06 will still be swaging the bullet down at the 1" mark, then the peak pressures add, and VIP's dad got spliters.

And I learned from Hot Core that Che lives.

[ 05-27-2003, 04:53: Message edited by: Clark ]

Clark, just curious how you figure my father got splinters? No mention of my father was made by me or anyone else. I guess that by swaging a bullet of larger diameter doesn't require any more energy than one of a smaller diameter in the same barrel. If it does require more energy, than where did it come from since shooting larger diameter bullets actually decreases chamber pressure? Could this be why the chamber pressure was less? Because the energy of the expanding gases went into swaging the bullet? Was there anymore heat caused by the friction of a longer bullet. Remeber the barrel didn't get any bigger so the bullet must have gotten longer creating more bearing surface. Just wondering. Thanks.

VIP,

My mistake, there were too many characters for my careless brain to keep straight.

I think the best way to look at it is not energy, but pressure required to swage a bullet down to a smaller diameter. This will become a local maximum in the graph of pressure vs time. One might ask if it would also cause a delay, but this does not seem to have a big effect in my experiments This is probably because I have done all my swaging a few thousandths of an inch after the bullet started moving. To calculate the swaging contribution to pressure from swaging I looked at the strength of lead and copper in compression and the bullet's cross sectional area [normal to axis] before swaging. This is about 5 kpsi that adds to the pressure curve about that point in time.