Chamber neck diameter?

[Boisblancboy]

I just received a copy of the .308 reamer that will be used in my new f class gun. My loaded unturned outside neck diameter is .334. The reamer print shows .343. So my question is .009 too much clearance? I thought it should be tighter than that.

I also want to turn the necks which of course is going to increase that clearance by a thousandth or two.

Thanks for the advice in advance.

 

[racesnake]

I've found that different brands of 308 brass have different wall thicknesses. Winchester seems to be the thinnest and Federal the thickest. Seems like I've bought a handful of neck bushings to get around this problem! Maybe you can get closer by trying the different brands.

 

[jscandale]

.0045" per side is a bit on the heavy side. Ideal is about .002-.003" per side. I'm sure you will find some bench guys here say that .001" per side is too much.

JS

 

[JRS]

Sounds like a SAAMI spec reamer?
Did you want something tighter? If yes, who ordered the reamer? Free bore length?

 

[fdshuster]

boisblancboy: There is an excellent article on outside neck turning in the Summer 2012 issue (#83) of Varmint Hunter magazine, beginning on page 44, titled "Neck Turning".

Sounds like you have a typical factory spec chamber which does allow for a fair amount of clearance between the case neck and the chamber neck. Lapua brass is the thickest of all, and to use it would reduce the clearance by a few thousandths. Is it worth it ? That's for you to decide.

A quote from the before-mentioned article: " When cartridge necks are turned in factory rifles the expansion of the thinner necks requires that the neck be reduced more when it is being resized by conventional reloading dies. Unless they are annealed, case life will be reduced. The probability of neck splitting also is increased".

 

[Boisblancboy]

Sounds like a SAAMI spec reamer?
Did you want something tighter? If yes, who ordered the reamer? Free bore length?

I was hoping for something tighter because I don't see any drawback to having it tighter either.

I'm having my gun built through Pierce Engineering and they already had a reamer, but I just got the print.

Freebore is .260, for the 230 gr bergers.

 

[shortgrass]

The best thing about a reamer that has a neck section that is too big, is that it can be made smaller!

 

[Boisblancboy]

The best thing about a reamer that has a neck section that is too big, is that it can be made smaller!

I wouldn't mind hearing about that?!

 

[Forum Boss]

The best thing about a reamer that has a neck section that is too big, is that it can be made smaller!

I wouldn't mind hearing about that?!

What he means is the reamer can be ground down.

By contrast, if the reamer neck diameter is too small, you can not ADD metal to make it cut a larger diameter hole. I encountered that problem. Ordered a .272 6mm reamer, and what arrived was a .270. I can't make the reamer bigger. If it was a .274 it could be ground down to .272.

 

[Boisblancboy]

The best thing about a reamer that has a neck section that is too big, is that it can be made smaller!

I wouldn't mind hearing about that?!

What he means is the reamer can be ground down.

By contrast, if the reamer neck diameter is too small, you can not ADD metal to make it cut a larger diameter hole. I encountered that problem. Ordered a .272 6mm reamer, and what arrived was a .270. I can't make the reamer bigger. If it was a .274 it could be ground down to .272.

Got ya. Now of course the owner of the reamer would have to agre with that.

So do you guys think .009 is too much clearance?

 

[fdshuster]

boisblancboy: One of the big advantages of a tight, fitted neck is extremely long case life.

As an example: A chamber neck diameter of .262", loaded round neck diameter of .260", and using a neck bushing of .259" diameter. The brass is being "worked" a total of .003", and split necks are unheard of. I have Lapua ppc & BR brass with 40+ loadings and they continue to be as good as when new.

In addition to that, since the case is locked up tighter in the chamber, a higher degree of accuracy can be expected.

I'm only one of many who spec out & purchase their own chambering reamers, with the exact dimensions we want, not what the gunsmith happens to have available. At a cost of around $138 if there is no need for future use, they can be sold for a good portion of their cost. A small expense when you figure the cost of a new high quality barrel blank ( $320+) and the gunsmith's cost to thread, fit & chamber, approx. $200.

As mentioned reamers can be reground to make smaller dimensions, and I've had that done also, at a typical cost of $25.

Don't accept an all important chamber just because the reamer is available, then be dis-satisfied with the results for the life of the barrel.

 

[Scott Harris]

.343 is SAAMI minimum....if you want the flexibillity to use any brass, I'd leave it as is. If you are confident you will always be using thin brass like Winchester, then get a smaller neck that leaves .004-.006 clearance. Then again, I don't think anyone claimed you will get better accuracy with a tighter neck, just longer brass life ....if you don't anneal.

 

[Bart B.]

The most accurate .308 Win. rifles I know of all have SAAMI spec chambers or a virtual equal thereof. No tight necks. .344" or so with even thin-walled 150-gr. WCC58 match cases with loaded round neck diameters of .333" (lots of clearance, huh?) easily shoot sub 1/2 MOA groups at 600 yards. Sierra Bullets uses such chambers testing 30 caliber match bullets from .308 Win. cases; the best of them stay under 1/4 MOA at 200 yards.

Note that all rimless bottleneck cases center perfectly up front when they're fired. Necks are perfectly centered in the chamber regardless of how much clearance there is around them. Even a .243 Win. round will center its bullet perfectly in a .358 Win. chamber and barrel when the firing pin strikes it.

The key to well centered bullets in chambers is getting the case neck well centered on the case shoulder. Full length sizing typically does this better than neck sizing. You gotta hold the case body in place while sizing the fired case neck back down. I'm convinced (after using all sorts of sizing dies and measuring neck runout on shoulders) full length dies with bushings to this the best.

Regarding small chamber necks giving longer case life, a friend and I did a test some years ago. We full length sized a single .308 Win. Federal brass case then loaded 42 grains of IMR4985 under 165 and 168 grain bullets. Shot it in SAAMI spec chambers with diameters about .344 inch. Loaded round neck diameters were about .336. With max loads, I got 47 reloads on my case and my buddy got 75. That's when we both run out of the IMR4895 powder from the can I split between us. His rifle was clamped in a machine rest; all 57 shots went into .3 inch at 100 with the 168 HPMK"s. I just reloaded mine and a buddy shot them from a bench with the match rifle resting on bags; he shot all inside about .8 inch with Sierra 165 SBT's at 100 yards. We weren't testing accuracy, but both chronographed the loads. Velocity spread across all shots fired was about 30 fps. No annealing of case necks whatsoever. Had to trim the cases back every 8 to 10 shots as they grew about .001" each time they were full length sized having shortened about .0006" when fired.

 

[Boisblancboy]

Thanks for all the advice. I guess I wouldn't have thought that more clearance in the neck wouldn't affect accuracy.

 

[fdshuster]

Boisblancboy: As others have said, and I agree, it's more a factor of much longer case life. That's the reason I'm able to get over 40 reloads on the ppc's and BR's.

That alone makes it worthwhile for me.

 

[Boisblancboy]

Boisblancboy: As others have said, and I agree, it's more a factor of much longer case life. That's the reason I'm able to get over 40 reloads on the ppc's and BR's.

That alone makes it worthwhile for me.

Yeah I completely understand getting longer case life. I will be annealing which will help and hope to have an annealer before too long.

Though I think this time I am going to go ahead with the sammi spec'd reamer. Especially since this is my first build. Though when it is time to rebarrel I think I'll have a good idea of what I want to try next. From neck space and freebore, considering who knows what bullet I'll be shooting.