Hand Finish Reaming And Chamber Throat Dimensions

[RifleMan]

This isn't so much a problem for my slowly-coming-along Yugo Mauser Scout project, but more a question of general interest to me. I respect the experience of you guys here who've been there and done that, and maybe you have the answer(s).

 

It occurred to me that accuracy really starts at the chamber throat, and through some reading I've done, it seems that most factory-new chamber reamers start off about 0.001" larger than the bullet diameter. To me, it seems that this would allow a bit of slop in the throat as the bullet unseats from the cartridge mouth and jumps to the rifling. A freebore this large *could* allow a bit of yaw (is that the right term?) and the bullet could start onto the lands a bit cocked. If the bullet itself has a relatively long cylindrical portion, maybe the problem is automatically corrected (to an extent), but if it's got little then it could well be spat out the muzzle with a wobble that would dramatically affect accuracy at the target (especially for long range or super-precise shots).

 

Anybody got any more to add to this? At this point, I'm not a good enough shot that it's likely to matter, but I think it's an interesting subject, and I wonder if having a reamer ground down a bit tighter might make sense, assuming that the shooter keeps getting better.

 

Thanks,

Nathan

[Horsefly]

It seems to me,that if the bullet explodes out of the case with a bit of wobble,goes down a good barrel for 22'' or 24'',and still comes out wobbling,you got the wrong bullets.The riflings should stablize the bullet,and then the crown gives it a good even send-off.You ask a good question,and this is the place to get good answers,so maybe we can learn something when the experts wake up.Jerry

[Spiris]

If you use the same type of brass with consistent neck thickness and the same or measured consistent bullets, then closer tolerance reamers, called match reamers, are important in the quest for ultimate accuracy. In my world, there are variations in manufacturers brass and bullets. Using factory spec reamers, but paying attention to the other variables for accuracy that I can control is the way that I approach rechambering. Here is a great article from someone who makes some of the most accurate barrels in the world.

 

http://www.riflebarrels.com/articles/barre...el_accurate.htm

 

Spiris

[odies dad]

Another issue to consider is that the case neck needs some room to expand to let go of the bullet. Too tight a throat will drive up chamber pressure drasticly.

The rifling and barrel length should stabilize the bullet like Horsefly said.

[RifleMan]

All good points... and a great article by Dan Lilja. Thanks guys. I'm still interested in learning more specifically about the throat diameter, as I've personally observed a "step" or "lip" just prior to the rifling in some factory barrels, and it seems to me that this is an indication of too loose a throat. Other barrels may have a smaller step, that I couldn't see with my eye, but they appeared to be roughly the same diameter as the groove (and yes, I know that the nominal groove depth is not always the actual depth).

 

Here's another article I ran across - seems interesting, but I don't know if it applies only to TC barrels or any. Seems to me the principle should be applicable to any barrel you're likely to run into.

 

http://www.bellmtcs.com/FAQ/ChamberThroats101.htm

[MarkTX]

Interesting article. Thanks Nathan.