Re-chambering a semi with a tighter chamber

[Quicksilver]

Let me start with a story.

Plenty of times I have taken my AR-10 out to 400-500 yards and with handloads, the gun performs just fine. Won't shoot bugholes, but will keep fist sized groups and definitely save your life if it ever needed to do so.

Finally, I got to really "stretch" her out to 800, 1000 and 1,200 yards. Beautiful day in Texas, cloudy, no mirage till 11 AM (amazing!!!!) and wind pushing right to left from 2 O'clock at 3-4mph steady. 65 degrees, 85% humidity, 29.91 baro... literally couldn't get much better. Stayed like that from 7 AM to 11 so we had PLENTY of time to get in nice shots.

Anyway, my friend has a Rem 700 SPS "tactical" or something like that. Nice rifle for the money, and no mods- just a SWFA scope (spelling?) and a cheap bipod and bag.

My groups at 800 could be contained in a spread hand, his a fist.

My groups at 1000 could be contained in a 12" circle, his about 8"

At 1,200 yards, I was happy to hit the 2'x2' steel target, he missed 1 "ring" out of 10 shots.

I fired his rifle just to make sure I wasn't having a bad day- and I wasn't. Guess my semi just doesn't get out there.

So, I've asked Armalite the specs on the chamber before and was told "none of your business" so of course, I just check my fired brass and go on about my business. I kept a few pieces of brass from my friends gun to check his in comparison to mine and found that my brass loaded is .3325 and after firing, is .345 which is ALOT of neck expansion. His is .3325 loaded (same winchester brass) and is .339 after firing.

After reading all this, your saying "great, what is the question!?"

Can I use a "bolt action" standard reamer (not a "match" reamer that requires neck turning) in a semi-auto that never sees dirt or combat? Its really just a bench gun that gets to ride along alot- plus, I'd never attempt to shoot any gun after dropping it on the ground here in the states unless its life or death.

I know that the semi chambers are cut large to accomidate dirt/debris, but leaving that stuff out of the equation, can I "tighten" up this chamber (with a fresh barrel blank of course) and still have a reliable rifle?

 

[mallrat]

Going with a new barrel and having it machined to minimum chamber clearance, you could probably hit it again to open it up, if needed later. Just run the reamer in a bit deeper, not messing with the extension threads/shoulder. If you have the means, I'd definitely give it a try. Keep your old (current) barrel in the back of the safe or parts drawer, worst case scenario, you can swap it back in.

 

[308sako]

There is no simple answer for you as to re-chamber or setback a semi the barrel extension would need to be timed to align the gasport to the extension once again. Then the length of the gas tube is wrong...

Maybe you should stop by at Bill Wiseman's Bill Wiseman as he is in your area and is actively working with gas rifles and accuracy at this time. Specifically he has a test fixture which the industry giants are all clamoring for.

 

[Oryx]

Also, if you don't pay a lot of attention to brass prep, specifically neck thicknesses, you can run into some chambering issues if you create a tight neck chamber. A bolt gun is a little more forgiving than a gas gun clambering in the blink of an eye.

 

[Quicksilver]

What is the standard diameter of a bolt action .308 rifle reamer vs a semi- .308 reamer?

 

[Friedfish2]

I'm in the EXACT SAME SITUATION as you are. I have an Armalite AR10 and it shoots slightly better than 1 MOA with the factory 20" barrel. The barrel appears thermally stable except it will only begin grouping better when I seat the bullets long and single load them into the chamber. I like the rifle and want to stay with it.

I believe the loose neck, throat and length of freebore doesn't help. I believe that a long throat can shoot very well, but I believe the neck diameter and freebore diameters must be minimal to maintain control. The factory recommended magazine maximum Cartridge Overall Length of 2.83" or 2.84" might be nice for the heavier bullets but doesn't leave much bullet base inside the case when loading a 155 Palma or 165 gr. soft point, or was that when I was loading to the lands? I can't remember….

I wonder if the standard Bisley reamer or one of the Palma reamers (2156) might work for me. The Palma reamer has a very short throat. Can these be used in an autoloader?

I'm almost but not quite ready to call Dave at PT&G to order my reamers, according to his recommendation, of course….

am I still on topic?

and then you have to find someone to cut the new barrel….

 

[sinister]

A custom barrel (with a tighter than factory Armalite neck) is the perfect choice, remembering that a semi needs a tiny bit more slop due to the bolt and carrier seating cartridges with more force.

You absolutely CANNOT go wrong using Bill Wiseman as your gunsmith. The Marines sent him to the Navy's Machinist A-School. Bill produces his own barrels (he got his start after buying Gale McMillan's barrel operation). He produced many of the original M40A1 rifles while a machinist at the Precision Weapons Shop at Quantico, and was one of the US Olympic Team gunsmiths.

I was always shocked to learn he was planting roots in Aggieland. My every day carry M1911A1 has the same 45 barrel he fitted for me back in 1980 or 81.

 

[tipper]

Here is a list of reamer spec's

.308 Palma Match
Shank 0.4370
Base Diameter 0.4690
Shoulder Diameter 0.4550
Neck-1 0.3400
Neck-2/Case Mouth 0.3400
Freebore Diameter 0.3085
Pilot Diameter 0.2970
Min. Chamber Length/Base-to-Case Mouth 2.0200
Base-to-Shoulder 1.3540
Neck Length 0.3090
Freebore Length 0.0750
Rim/Belt Thickness 0.2000
Shoulder Angle 20*
Throat 1.5*

.308 Win commercial
Shank 0.4370
Base Diameter 0.4700
Shoulder Diameter 0.4550
Neck-1 0.3462
Neck-2/Case Mouth 0.3442
Freebore Diameter 0.3100
Pilot Diameter 0.2990
Min. Chamber Length/Base-to-Case Mouth 2.0250
Base-to-Shoulder 1.3540
Neck Length 0.3210
Freebore Length 0.0900
Rim/Belt Thickness 0.2000
Shoulder Angle 20*
Throat 1*45”

7.62 NATO
Shank 0.4670
Base Diameter 0.4725
Shoulder Diameter 0.4560
Neck-1 0.3460
Neck-2/Case Mouth 0.3460
Freebore Diameter 0.3110
Pilot Diameter 0.2990
Min. Chamber Length/Base-to-Case Mouth 2.0250
Base-to-Shoulder 1.3560
Neck Length 0.3200
Freebore Length 0.1500
Rim/Belt Thickness 0.2000
Shoulder Angle 20*
Throat 2.5*

M852 precision military chamber
Shank 0.4370
Base Diameter 0.4700
Shoulder Diameter 0.4550
Neck-1 0.3430
Neck-2/Case Mouth 0.3430
Freebore Diameter 0.3095
Pilot Diameter 0.2990
Min. Chamber Length/Base-to-Case Mouth 2.0200
Base-to-Shoulder 1.3540
Neck Length 0.3120
Freebore Length 0.0750
Rim/Belt Thickness 0.2000
Shoulder Angle 20*
Throat 1.5*

The closest too the Wylde is the M118 Obermeyer Match. This is what little info I could find on the Obermeyer.
Base Dia. .473
Shoulder Dia. .455
Neck @ shoulder .344
Neck @ mouth .344
Neck lenth. I assume is in the middle of 7.62 Nato and M852 at .315
Freebore Dia. .3085
Freebore lenth .3085
Pilot Dia. .299
Base to case mouth. 2.02
Base to shoulder. 1.354
Head lenth from base. .200
Shoulder 1/2 angle. 20*
Leade Angle. 2.5*

 

[sinister]

The reamer you (or the gunsmith) chooses should mate up with the bullet and style of shooting you will use the most.

A neck and freebore optimized for a 168 will not be the same as for 155s/175s and heavier (180 - 208s, etc.), and different manufacturers use different bullet ogive shapes (secant, tangent, and hybrid).

 

[flatland1]

I bought a 308 AR bbl from WOA several years ago for a DPMS AR build. Had no luck getting it to shoot worth a hoot at 600 until I spoke with Dave Kiff about it. When I told him that John H. uses an Obermeyer Match reamer, he perked right up & told me that was probably most of the problem. Boots' 308 reamer uses a 2-1/2* leade angle, while Dave prefers a 1-1/2* leade. He advised me to run my Bisley reamer by hand just enough to re-cut the lands in the leade to the 1-1/2* angle, which I did. You may not believe it - I was very surprised myself - but just doing that, the rifle went from not being able to keep shots on a full-size steel IPSC target at 600 to printing groups of 1 MOA or less with Sierra #2155s. I know that's not a whole lot better than what the OP was asking about, but it was a huge improvement for this bbl.

 

[The Gump]

If you know someone with a pile of ar 10 bolts and a headspace gauge set you could keep trying bolts until you find one that closes on a zero but won't close on a +.001.

 

[The Gump]

Never mind. I misread your post.

 

[arnie19]

Boy all of these chambers seem to be very big at the neck ,from .010 to .015 or so .I would think a match chamber would be about .005 over a loaded round . Arnie

 

[tipper]

Lapua, LC, & most mil spec brass will will have a neck OD of .336-.339.
The Freebore Diameter of 0.3085 is the max I would like to have, as the Freebore Diameter gets larger the more gas pressure is lost before the bullet seats in the bore,

 

[Matno]

I haven't measured it, but I'm fairly certain the chamber on my dad's Les Baer 308 is tighter than my Armalite AR-10. Lives up to Les's accuracy guarantee with match ammo (1/2 MOA) but jams every 3rd round or so if using cheap ammo. My Armalite has yet to have a single malfunction with any ammo I've run through it, but I'm lucky if I get close to MOA. It's a trade-off.

 

[Quicksilver]

I'll track down Bill Wiseman and pay the price. Although, being here in Aggieland sure helps- also, just a shout out- Barry at Burdetts does good machine work. He may not know everything about trick reamers/bullet combinations, but still a good guy.