I'm worried about my chamber in my .308 auto loader being cut too long

[JerryB]

Hey guys,

I bought a new AR-10 (great manufacturer, shall remain anonymous though). This is my first .30 cal. rifle. I bought my modified .308 Winchester case for the Hornady length gague, and used it as I would with any other chamber. I'm a machinist by trade, I know how to use my measuring equipment correctly.

The stated total cartridge length is 2.800", and magazine length allows for that. However, upon using my gauge, I have found that the three bullets I am using must be loaded to the following total cartridge lengths to contact the lands:

The first number is the complete length required for the bullet to contact the lands. The second number in parentheses is how much over said number is from the 2.800" max length. AKA, jumps to lands.

Hornady 150 gr SST: 2.922" (0.122")
Hornady 175 gr A-Max: 2.920" (0.120")
Sierra 168 gr Matchking: 2.892" (0.092)
Nosler 125 gr Ballistic Tip: 2.966" (0.166") (only the boat tail protrudes in the case neck, impossible to seat up to lands. )

Seeing such a large jump to the lands is an alarming situation to me. Upon purchase I picked up a box of 168gr Federal Gold Medal Match and shot through the box (which was loaded to 2.800"). With the two five shot groups I shot after sight in, it produced a 0.786" ctc group and a 1.192" ctc group. I am obviously limited to 2.800" max, to feed from the magazine. Knowing these numbers, I don't feel safe loading or shooting this rifle.

Again, these stated numbers are NOT measurements to ogive, they ARE the overall cartridge lengths to contact the lands.

I doubted my process after seeing the numbers, so I checked a few other rifles and my methods and madness produce the same numbers as always.

If I question the integrity of the barrel, I will certainly not shoot it.

Please gentlemen, shed some light on what the deal is here.

Thanks for your time,
Jerry

 

[Justin1098]

If I am understanding you correctly, you have more freebore than you want.
In my experience, its normal to be limited by magazine length in an ar platform. You would have to get a custom barrel chambered with a short throat to be able to touch the lands.

That being said, all is not lost. You don't have to touch or jam the lands to have accuracy. Even in my 6br, I load sierra matchkings to jump about .1 inch. Matchkings usually like a jump. You may have trouble with vld bullets but Berger is now saying they like to be jumped .030 or more in some cases.

As for the integrity of your barrel, longer freebore reduces pressure making it safer. That's probably why oem barrels have long freebores in semi-autos.

You shot MOA with GMM's. That's promising.

 

[DennisH]

Some of my most accurate loads are with the bullets being .050 off the lands.

On my Savage 22-250 I tried to measure a 55grn V-Max with my Hornady COAL tool. Impossible, the bullet didn't even start to go in the neck. I don't have a clue how far the Factory loads are off the lands, Fed 22-250 Nossler Balistic Tip 55grn. They are very accurate and it's nothing to hold 1/2 MOA @ 200yds. 100 yds shoot a tight cloverleaf.

 

[M14AMU]

Sad to say but when you buy a factory rifle, made for the average Redneck that just wants MOD aka Minute of Deer.....you're very limited on options!

 

[Steve Blair]

I think you have ~.170" freebore. That is not excessive for a factory rifle, particularly an autoloader. Try shooting it with magazine length loads and see what you get. You may be surprised. It is not dangerous in any way. If it was, every Weatherby on the planet would be, too.

 

[XTR]

Those are pretty normal numbers esp considering a factory chamber and you are loading to mag length, and you're limiting to 2.80.

In my standard R700 mag loaded rounds are limited to about 2.835 if I remember correctly, in my H&S DBM I'm limited to to 2.850, I think there are some of the AICS compatible systems that will get you out to about 2.90. Decisions associated with loading to mag length is a pretty common dilemma for guys who do the tactical shooting and have to use mags but want to use long high BC bullets.

 

[kraky]

I have a factory rem vtr with at least that much jump to the lands. I've got loads I've found for that gun that go .5 moa.

 

[brian427cobra]

May I recommend 175smk loaded at 2.800 pushed by 44ish of Varget, I assume you bought a DPMS or 1 of there rebranded ones, the 1-10 twist likes heavier bullets, my LR308 was a hammer for a auto loader, and no pig ever got up after being hit with the 175s.

 

[bigedp51]

You have an AR platform designed for combat conditions, the longer throat allows the chamber pressure to build slower and reduce bolt thrust. A wet or oily chamber can double bolt thrust and damage the bolt and the receiver bolt lug recesses.

On top of this your AR is "tuned" for its pressure curve with its longer throat, gas port location and gas port diameter. As an example if your gas port pressure is too high the bolt will start to rotate before the cartridge case stops gripping the chamber walls. This will cause brass smears on the back of your reloads and may cause chambering problems.

In short the long throat is normal and was put there for a reason.

 

[243winxb]

SAAMI Standards 308 Win.

http://www.saami.org/PubResources/CC_Drawings/Rifle/308%20Winchester.pdf

 

[JerryB]

Thanks for the quick responses and information! I was unaware and leary of this situation. Being that this was intended and normal, i will say that it is a Rock River Predator HP I picked up on a whim for a coyote rifle. The reason I was worried is that in measuring my RRA NM rifle (I know, different gun/caliber, different animal) I am only jumping 0.010" with a 2.26" OAL. My other bolt rifles share similar dimentions.

I see now that I was unaware of the freebore's safety attributes. It appears that many of you are having success with similar chambers and loads, I will revisit my setup and continue on. I really appreciate your help in resolving my situation guys, thanks! It is a substantial relief. 8)

243winxb, thanks for the link, very telling!

Jerry

 

[jonbearman]

I load my rounds at magazine length (168 hornady match) and at 100 yds have shot well under 1/2 minute of angle.The trick is dont load them real hot.A medium load will do less battering to the gun.

 

[bigedp51]

Look at the following differences between the .308 and 7.62 chambers.

Base diameter of chambers, the 7.62 NATO is .0025 larger .
Freebore diameter is .001 larger for the 7.62 NATO
Throat angle 1 3/4 for the .308 and 2 1/2 for the 7.62
Freebore length .090 for the .308 and .150 for the 7.62 NATO.

NOTE #1: My Savage bolt action .223 has "LONGER" freebore than my 5.56 AR15.
NOTE #2: Many commercial .308 rifles have longer freebore than the 7.62 NATO chamber listed above.
NOTE #3: The chamber freebore must be long enough that a company Lawyer will fit inside the chamber and thus prevent law suits.