Finish reaming without a lathe - Good or Bad?

[hkim]

I have a barrel that was improperly chambered by a gunsmith,.308 Win), having the mouth end undersized by 0.006". Cases seize in the chamber, making extraction difficult at best. I am considering opening up the chamber using a piloted finishing reamer with close fitting bushings, turned by hand with a tap handle. I don't have a lathe to do this by the conventional method. Anyone with experience on this who can share their advice?

Thanks

 

[Joel]

I recommend having the original smith fix it.

When you state the “mouth end undersized by 0.006". I am taking that to indicate that you have a tight neck. This can be solved neck turning your brass. If you want to open the neck I believe Dave Kiff of Pacific Tool and gauge makes a neck only reamer.

If you are stating that the shoulder is tight that is a reamer problem. This could be solved by hand reaming, however, you have to be sure that you don’t change headspace. This is not an easy task if you aren’t familiar with chamber reaming. Hand reaming can be done successfully in trained hands and it can turn a reparable situation in a pile of junk in untrained hands.

My real concern is “Cases seize in the chamber, making extraction difficult at best”. If extraction is difficult on an unfired round you are correct that the neck or chamber it tight, however, if it chambers OK then extracts difficult you have other issues.

How does the fired and unfired brass look after being chambered?

How did you determine that it is 0.006” undersize?

 

[hkim]

Gun is an AR-10 with a custom 20" PacNor barrel in .308. Original gunsmith already goofed up the barrel contour and gas port location from my provided drawing. I will not give him any of my business again.

Chamber dimensions were taken from a cerrosafe cast measured with a Mitutoyo blade micrometer. These are as follows, with corresponding SAAMI minimums in parantheses:

Mouth 0.46525",vs. 0.4714")
Shoulder 0.4547,vs. 0.4551")
Neck 0.34525",vs. 0.3442")
Freebore 0.3085",vs. 0.310")

The mouth and freebore are under SAAMI spec, while the shoulder and neck are about SAAMI minimum. Throat is very short compared to the stock AR-10 chamber. Chamber was polished and has no scratches which would cause extraction problems.

Unfired rounds hand cycle easily. Live rounds hand cycled are not engraved at the throat, so the rounds are not contacting the lands when chambered. When the gas is turned off by blocking the gas port with a dummy gas tube, the rounds will seize in the chamber and require a screwdriver to pry the bolt carrier open. Federal GM and Hornady TAP rounds have ejector marks on the case heads, maybe from high chamber pressure. When cycled in semi auto, the rounds will extract but with bent rims, suggesting the rounds seized in the chamber and extracted with difficulty. As indicated earlier, since the mouth is 0.006 undersize, radial clearance at the case head is visibly tight. A round inserted in the chamber will have almost no sideways play. The same round inserted into the stock AR-10 barrel, will have noticeable sideways wiggle. This stock barrel measured with a cerrosafe cast was at SAAMI minimum dims. I figure that adjusting dimensions to SAAMI minimum will solve most of the problems with cases seizing and high pressures.

The headspace is 1.631". Armalite's headspace limits are 1.631 to 1.633" so I have 0.002" to play with when hand reaming. Given this, how would you recommend doing the hand adjustment?

 

[rayjay]

I would seat some bullets quite a bit deeper and see what happens. If the unfired rounds cycle fine you may have some other issue causing high pressure. It might be good to chrono a few rounds also.