The minute a case won't extract or the rifle does not shoot well, the chamber is bad.
Throat erosion and muzzle erosion certainly effects accuracy. But it takes 4000-5000 rounds for a 308. Around 1200-1500 rounds in a 6.5mm. I heard 800 rounds in a 243.
With reloads, most extraction problems are due to over pressure rounds, neck sized rounds, insufficiently sized cases.
I had one experience with a "bad chamber". I had a Douglas 1:10 installed on a pre 64 and in 30-06. It tested well, shot well with 168's. It did not do well with any bullet heavier than a 168.
I sent the thing off to another gunsmith who rechambered it to 308 and used a "floating reamer".
The rechambered Douglas barrel shot every bullet weight well, and I just recently removed the barrel because it was shot out.
Gun drilling does not necessarily produce a straight hole. The drill moves up and down, left and right. The hole may be in the center of the ends of the barrel blank, but can be anywhere inbetween.
If a straight reamer is stuck in a curved barrel hole, the chamber will not be round. The gunsmith who used the floating reamer claimed that reduced out of roundness.
Military barrels, hunting rifle barrels, these are cheap barrels. Military barrels are given to lads who have spend more time marching up and down the square than they have shooting. Our Iwo Jima Vet had exactly 20 rounds before he was shipped out into a combat zone, my Uncle, in the 101 Airborne, had exactly 7 rounds of familiarization before being dropped in Normandy.
There is absolutely no need for cannon fodder to be issued expensive target grade rifles. All that matters is that sufficient quantities of reliable rifles are issued to these doomed lads so they will have a weapon in their hands before they, and the weapon, are blown all to hell. I understand 90 days was about average in WWII.
In war, lives are cheap, and the barrels even cheaper.
