doublegunshop.com - home Forums General Discussion Classic & Custom Single Barreled Sporting Rifles @ doublegunshop.com A pressure problem or something else?

In the USA, max pressure of the 8x57 is held to a lower standard (by SAAMI). This is done as early J bored had a .318" bore. If you fire a .323"JS bullet, it will swage down to the smaller bore, but 99.99% of the time no damage will occur.

In the blown rifle, the bullet was still in the barrel. That just doesn't happen. 100,000+psi (enough to blow up a barrel) means that bullet isn't going to stop. No way, no how.

He can claim the barrel was clear, but IMO, that barrel had an obstruction.

I assume you mean 0.1mm steps, in which case, the best that can be said about a bore which will accept a 7.7mm pin and not a 7.8 mm is that the bore is larger than 7.7mm and smaller than 7.8mm. Use of any gage assumes that the operator is skilled enough to use it properly.

Standard pin gages are 2" in length, to use properly up to the full length of the pin should enter the bore. This precludes installing it at an angle. Standard pin gages are furnisheed in master sets with pins in 0.001" increments, permitting measurement to +- 0.001". If this is not sufficient for the application, individual pin gages can be purchased in 0.0001" increments. Ideally, for measuring a bore, a set of gages on either side of optimum would be purchased to determine the actual bore diameter.

Pin gauges, .0001 tolerances, "precisely square to the bore"...

What happen to slugging the barrel with lead?

This week I'm going over to a buddy with a lathe and we're going to turn a .349 pin, if as the reborer stated he does his Whelens with a .349 bore it should fit, if it doesn't we'll try a .348, .347, etc.

Will report back, shouldn't be hard to tell which it's closer to, the .349 claimed or the .344 I measured with lead.

It would be much simpler to find someone with a proper set of pin or plug gages. They are a common item in many machine shops. If you were located near me, I would be happy to help.

Thanks for the offer, this being the heart of blue state territory not a lot of machine shops.

RecoilRob,
It doesn't have that much to do with a tight bore. What it shows is that the case is expanding enough to release the bullet, rather than restricting it, thereby driving the pressures up. If the bullet is not restricted from starting to move, a slightly larger bullet will not unduly raise pressures. On the otherhand, if something restricts it, such as case neck/bullet diameter too large for chamber neck or too long case jammed into leade, pressure must rise enough to overcome the resistance before the bullet can move. This results in a higher peak pressure. If the bullet is able to move without undue restriction, before entering the barrel, the volume of burning gasses is increasing which moderates the increasing pressures. By the time the pressures peak, the bullet has been sized to what ever the barrel diameter will allow. This occurs by the time the bullet travels the length of it's shank into the barrel. This description of what happens may not be precise, but is how I understand it. This is not something I discovered, it comes( as I understand it)from experiments done while developing the German 8x57 IS cartridge from the 8x57I; before WW1.
Mike

Agree.
While the investigation into bore diameter should continue, a chamber cast would also be a very good idea.

I asked about that early on and was told not necessary since I have the specs on a fired case.

JC,
I did mean 0.1mm steps. A correctly sized pin will only enter the bore if it is properly presented and this does depend on the skill of the mechanic. So does the proper use of small hole gage depend on the skill of the mechanic. There are a great many more small bore gage sets around than pin gage sets, and they are considerably cheaper. Furthermore, one set of small bore gages will cover all the more common barrel bores, but it would take two pin gage sets to span the diameters. You would need a precise measuring instrument to use small bore gages, whereas the pins themselves are the instruments. On the other hand, we all need a precise measuring instrument anyway, so that is not really an extra requirement. Pin gages come in .001" steps, but common micrometers read to .0001" on the Vernier. I guess what it boils down to is we just use the best tool we have, the best way we can.
Mike

Recoil Rob,
It is never a mistake to make a chambercast. You can get good diameter measurements, but length measurements are harder. A chambercast from a bolt action rifle doesn't have a precise point to represent the case head.
Mike