For a .308 Win. I would like to polish a rough chamber. What is the best way? Not too terrible bad, would like some advice. Or Do I need to seek a Kreiger Barrel? Thought about it any advice would be appreciated. It's for an M1A!

King,

The best way to polish the M1A chamber would be to pull the barrel and run it at high speed in a lathe, and using a mandrel with some very fine emory cloth attached and using some oil, run the mandrel rapidly in and out of the chamber producing a fine "cross hatch" type finish like that in an engine cylinder. Do not leave the abrasive mandrel in one spot too long or you will jug the chamber.

For a mandrel, I use a .250 diameter rod that is slit at the end to attach and wrap the emory cloth. Be very careful as you work near the shoulder so as not to alter the headspace by removing too much material. The idea in polishing is to do just enough to remove or reduce the problem, not alter the chamber dimensions...

Good luck,

Malm

Is there a lapidary shop near you? You can take a chamber mop and make a thin paste from 1000 grit or higher with thread cutting oil. Do as Malm said and don't leave in one spot. I have done this with dies and works well. After you brush up with the grit use jewelers rouge to make like chrome plated. This is used for the final polish on stones. Will work wonders on metal. Use on an old knife blade first to give you an idea how it works on metal.

I don't know if I am a believer in a highly polished chamber or not. I may need some convincing. I am a believer in a smooth chamber but not a highly chrome like polished one. Heres why.

A highly polished chamber does not allow the brass to share in the containment of the load that is placed against the bolt and lugs. The brass, unable to grip the chamber walls during ignition, slams back against the bolt face and transmits the majority of the energy to the bolt face and lugs.

I'm not sure, but I would think that if the case were better able to grip the wall of the chamber, that the load placed upon the bolt face and lugs would be reduced by a proportional amount. This could result in a little better safety margin when working with the high pressure loads that we do. Let the chamber participate a little more in the load sharing duties.

In a profound experiment by P.O.Ackley, while proving the capability of brass to withstand a certain amount of pressure, and to prove that the blown out case design of the improved cartridge would reduced bolt thrust, he took an old Win 94 and chambered the gun for the 30/30 Improved case. He fired a round and declared the brass case to be perfect. He then unscrewed the barrel 1 turn and fired another round. The brass case held and exhibited no signs of pressure but the primer backed out the distance of 1 turn.

Next, leaving the action out 1 turn, he oiled a case and fired it at this setting and the case exhibited a normal seated primer but the shoulder had moved forward indicating that the case had slammed against the bolt during firing proving that the presence of oil on a case enabled the case to lose it's grip on the wall of the chamber and slam to the rear increasing thrust on the bolt.

Then he turned the barrel out 2 turns, lengthened the firing pin so as to reach the primer and fired another cartridge. The primer fell out because the barrel was screwed out the thickness of the primer but there was no indication of pressure or damage to the brass. He then oil a round and fired it and the case seperated at the joint created by the 2 turns.

What this proved was that the blown out design of the improved case increased the grip on the brass which held the case in place reducing contact or thrust on the bolt, and it proved that the presence of oil on a cartridge case increases the rearward load and thrust that the bolt must absorb.

Later on he explained that a highly polished chamber could mimick the "oily brass" scenario, and that a slightly roughend chamber would allow the brass to grip the chamber walls which would reduce the load against the bolt in a similar fashion as the blown out design of the improved cartridge.

I don't know, it makes sense to me.

Regards,

Malm

G.Malmborg
Excessive headspace is still excessive headspace. Polished chamber allows for more uniform reloads once headspace is under control.

If that makes sense.

Later....

I have some data on Rocky Gibbs cartridges some were. Any way, seems no one could get the velocities he was.. His answer was no one polished there chambers the way he did MIRROR FINISH....

The way I understand this statement is.. The slick chamber allowed the case to slip all the way back to the bolt face making a perfect fit. (Crush fit)
In return, you would get the same result on that reload every time.

If the chamber is a little rough, the cartridge will grip on the wall, even if the case does fit the chamber totally, you may get a stick bolt feeling as well.


This all comes down to ones own interpretations..

Later....

Westerner, I can not agree with you on seal.. An imperfect surface is very difficult to seal, even with gasket maker....

[ 01-08-2003, 08:19: Message edited by: Mauserkid ]

Malm- I totally agree with you and would strongly advise anyone against polishing their chambers to a mirror finish. I polish mine with 400 grit using a method nearly identical to yours. I have a through the muzzel pressurized chambering system that invariably produces initial chambers that have a very fine surface finish. It's almost criminal to finally rough them up with 400 grit paper, but it's done for a very good reason. A slightly rough chamber surface definately allows the brass to grip the chamber walls and spreadout the pressure. -Rob

I do not polish chambers any finer than 400 grit for the same reasons already posted.

I polish chambers minimally because that is all that is necessary. I don't think the grip of the brass on the chamber walls contributes to containment significantly. I think Ackley was wrong in a number of his conclusions. So there! Regards, Bill.