What constitutes a "too loose" chamber?

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What constitutes a "too loose" chamber?

Postby Hoot » Sat Sep 03, 2011 7:51 pm

What is too loose a chamber in and AR?

My .300 OSSM cartridge is spec'd to hold 57.0 gr of water at 70F after shooting and before they are sized. Mine hold 56.8-57.0 after resizing and 58.9 gr after shooting and before they are sized. Is this indicative of a chamber that is too loose? That's a lot of work-hardening of the cases and despite annealing, they begin splitting necks after as little as 3 reloads. Most make it to around 5 reloads before they're so gored and gouged by the barrel extension that I usually replace them. I could have the resizing die opened up or buy a custom made one, but that's a little tough since I don't have access to the reamer print and besides, it probably wouldn't be cheap. If I had a decent lathe, I probably could do it myself by cut-and-try.

I've been muddling along with it so far, but can not get it to shoot as accurately as I would like across a range of powders and bullets. Not sure what the chamber has to do with that, but I do know that based upon the asymmetry of the expansion just above the web, the chamber may not be coaxial to the bore as well. If Olympic sold components for this caliber, I might just get a K8 Mag Barrel & extension to replace the GameStalker one on my rig.

Hoot
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Re: What constitutes a "too loose" chamber?

Postby Thom » Sun Sep 04, 2011 11:52 am

I don't reload for the AR, but with my Enfield & Springfield '06's I have always just neck sized. The chambers are so evenly matched that it has never been a problem swapping between them. Perhaps you should give it a test, back off your sizing die and just neck size your cases. It would certainly stop working the brass so hard.
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Re: What constitutes a "too loose" chamber?

Postby Hoot » Sun Sep 04, 2011 8:02 pm

Thanks Thom. I do the same thing in my bolt guns, but in autoloaders, I don't think we have that latitude. I have to bump the shoulder back a tiny amount each cycle, or the bolt wont go into battery. It's weird in that for a mil or two of bump, if you don't, it feels like you're trying to move the case a quarter of an inch to get it into battery. Two mils further and it literally falls into battery. Those dies definitely do the job, but the trip up and down seems to catch up with the brass around 5 reloads and they start splitting from the shoulders up to the mouth. I lose about 5 each cycle to loose primer pockets, which aren't the result of the resizing.

I just spent 4 hours on a busy 8 shooting bench rifle range that was making up for being shut the past three weekends for a major project that's now done. This is the beginning of the fall rush, where folks site in and function check their deer rifle(s) before it gets uncomfortably cold. A lot of stoppages while folks put up and took down paper. All for naught though. Both variations of the 140gr Hornady Monoflex performed equally horrible. Shooting at 5-up target papers, they looked like someone was testing their shotgun loaded with #00 buck at 10 yards. Gotta be the most horrible range session I've had since the 160gr Hornady FTX bullets early on. With this .300 OSSM GameStalker, I'm feeling like an old, tired, Don Quixote, realizing his futility. I own 5 other .30 caliber rifles. One is an old Century pre-ban G3 rebuild. It hadn't seen much range time the past two years, so I loaded up some rounds shooting the same Monoflex bullets, with three different powders, in 3 charges, it shot them all at worst 1.5 MOA at 100 yards. This is one sheet of what the .300 OSSM did on the target board next to them with the same bullets.

Image

Sadly, with the .300 OSSM, this is more the rule than the exception. I have spent a large part of this spring and summer trying to find loads that deliver reasonable accuracy at the speeds this caliber is capable of delivering. Not necessarily the lunatic fringe upper end, just somewhere in the zone. Midway and my local reloading shop are eternally grateful to OA for this upper. I can count the number of loads that deliver consistent good groups on one hand. So, I'm grasping at straws as to why so many of the loads I've tried do not shoot any better than they do. I'm probably one of the most fastidious reloaders you'll meet. That is reflected in the precision I can achieve in the other caliber uppers and other autoloading rifles I use, so I doubt it's my technique. I keep the gun very clean, so I can't fault myself for lack of proper care. I've tried three different $350-$550 class of scopes that deliver fine on other rifles, so I can't fault the optics. It seems to do well with the two brands of 180gr flat base bullets I've loaded it with and one 150gr flat base bullet at certain velocities, but only with certain powders though it doesn't like 165 or 168gr flat base bullets with any mix I've tried. Also, anything with a boat tail is doomed. Both of those 140gr Monoflex bullets were boat tailed.

So, once again, I find myself grasping as straws as to what the deal is. In my technical mind's eye, I realize the loose chamber is not the culprit. If that were the case, it would not shoot those few loads that it manages to shoot with reasonable accuracy. At first, I enjoyed just getting out and shooting it as I discovered it's potential to deliver a lot of power, but like in a soured relationship, where only one side gives, I now grow weary of shucking so many oysters and not getting enough pearls. :?

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