Anybody familiar with freebore in the Obermyer 308 AR chamber?

V

VaniB

New member
First the necessary disclaimer; AR rifles are "centerfire", and folks can and do fire it "benchrest". So, please, those guys that have a disdain for black rifles, please have patience with those few of us that use them for precision shooting, and resist telling me to go to a weekend warrior websight. For accuracy, THIS is THE place to go.:D

QUESTION: When handloading magazine length 308 cartridges, have you experienced that the Obermyer chamber allows you seat the bullet out closer to the lands? Or, in that regard will it really not be much different then the 308 Win chamber and have the same amount of excess freebore?
 
10-07-2010 10:37 AM Pete Wass

Originally Posted by Greg Culpepper

Actually Roy, the Obermeyer has a throat almost exactly the same as a 308 Win (.085 vs .090) but a 2'30" leade vs 1'30" for the Win. Chamber body dimensions are virtually identical and standard dies will work for both equally well.

Greg
 
I have prints for both and have worked with both.

The Obermeyer effective throating is considerably shorter than a standard 308 Win reamer for a few reasons (regardless of the only .005" difference of free bore length noted on the two different prints):

The free bore diameter of the 308 Win starts out at .310" in diameter (sloppy) vs .3085" (close) for the Obermeyer reamer and that make a big difference right away in where the bullet will hit the lands because the mild throat angles start from those different diameters); and

Coupled with that, the Obermeyer reamer has a two and a half degree throat angle vs the one and three quarters degree throat angle of the 308 Win (i.e. since the 308 Win free bore starts out bigger in diameter and has a more mild throat angle, the effective length of where a bullet hits the lands is a good bit longer for the 308 Win. than the .308 Obermeyer).

If you want to be a fair amount closer to the lands, use the Obermeyer reamer.

Example -

With a 308 Win reamer the 168 and 175 gr Sierra bullets I have hit the lands around 2.870 - 2.875" OAL

With the Obermeyer reamer, the same bullets hit the lands around 2.825"

Robert
 
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Thank you Mr. W for such an in depth reply. When folks post these kind of technical stats, they often don't take the time or effort to explain/translate how it physically works in the chamber. Your input was very helpful to me so that I now feel confident that this is the chamber I should seek for my next barrel to be made by Krieger.


Ummm.... While I've got you....... if you don't mind.....




......your opinion and validation would be invaluabe to me for a project I'm doing; I plan to have krieger cut an 18" barrel in M110 configuration with a 1-12 twist. It will be threaded for a flash hider, rifle length gas system, Obermyer chamber, .875 gas block, and a .850 muzzle. My goal is 5/8" 5 shot 100 yard groups using 167 -168 BTHP's. In your opinion, is this barrel heavy enough to deliver this kind of accuracy as a norm if I do my part? I only shoot 5 rounds in a single 15 minute relay, so I'm hoping that overheating shouldn't be a problem that way. Also, is an A2 style flash hider a deal breaker for accuracy? I see them on NM rifles....but NOT on other precision rifles like the Les Baer.
 
VanniB

I am not clear on what type of precision shooting you are intending to do with this.

First off, for precision shooting I would tend to stay away from a barrel threaded for a flash suppressor just because that typically means cutting the diameter of the barrel way down at the muzzle and threading it (i.e. I don't like doing that right on the end of the barrel where the bullet exits). Does that mean you cannot have a barrel with a flash suppressor that shoots well? No - but it may make for a less accurate set up if everything is not just right (including the flash supresssor - needs to vent gas 360 degrees and nothing not perfectly true and centered).

I would be wary of a rifle length gas system on an 18" barrel as that's real close to being non-functional for cycling with certain loadings unless the gas port is big (20" is fine, but when we go to 18" we typically go with a "mid length" gas system).

If you are planning on running off a rest or bench I would tend to run with the heaviest barrel I could squeeze into the platform. I just bought a few barrels for AR-10/LR 308 work ups off a bench and they were 1.2" dia for 3" then tapering over the next 2.5" to 1.050" then a .936" gas block, then .930" forward of the gas block (and if I had ready access to a 1" gas block, I would have done a 1" gas block diameter and .995" forward of the block). Now if you have to hold the rifle or carry it around the M110 configuration is fine.

I am not sure whether what I have said is helpful or not but those are my thoughts.

Robert
 
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I am not clear on what type of precision shooting you are intending to do with this.

Robert,

I do not engage in formal competition. 5 shot 5/8" groups at 100 yards is a personal arbitrary figure that must be met if I'm to be satisfied with this rifle. ( yep, I can accept an occasional flier to 3/4") I bring my AR rifles to the public firing range where I bench the rifle off of a Hart rest and each relay lasts about 15 or 20 minutes. If I can fire off that kind of a group, and then repeat it again during the next relay so I've established a proven load that delivers this kind of accuracy regularly, then I go home that day with a big smile which will last me at least well into Monday morning until I return to work.

I've had quite a few guys on the internet, including "slash" the buffer man, tell me that they've cut their barrels back to 18" with no problems. It seems in 308 that a .1 gas port is what works. I know my Fulton 18.5" barrel works fine....but I don't have with me at the moment to measure the gas port size. I'm not disputing what you say, as krieger also says that they will not warrant the functioning if I have them cut their 20" M110 barrel back. So obviously, there are factors and considerations involved with doing it (for example, I suppose the kind of loads being used) and I need to be careful before I proceed.

As you state, heavy barrels are much more inclined to provide the kind of accuracy I desire. But portability and weight of the finished rifle is a deciding factor to me, and it is for me to try and choose the minimum starting weight/heft for a barrel that will provide that accuracy. I'm inclined to go with the M110 cut to 18". But, I realize that it's a gamble that can forfeit me my $500, and I could end up dumping it and starting with another new barrel attempt all over again!

What you surely have convinced me in the last post is that I might seriously drop the idea of a flash hider. In my mind, the FH is an integral part of this military based rifle, and an AR should have it. (just my personal taste) But, if it might really make that difference as to whether or not I can get the 5/8" accuracy I want, then I might moderate my stubborn ideals and live without it.

Thanks so much for your input. Your time and patience is appreciated.
 
VaniB

An 18" barrel can work fine, but it's just close to the edge function wise with certain loads unless the port is big (and .100" for a port is getting big, and a big port has potential downsides for accuracy too, bullet is a lot more plastic passing a gas port than people realize and a smaller port minimizes disruption of bullet integrity as it passes the port). For bench shooting with an AR, many times you can get the best groups using a faster burning powder, and with the faster powders the pressure curve drops quicker and that is where cycling can become marginal with the 18" barrel.

Good luck with your build - always fun to hear of people doing new projects!

Robert
 
Robert,

Even your parting comment about port size and powder/pressure curves was informative. I didn't realize that the port size could have an effect on accuracy, but now that you've mentioned it....it does makes sense what you say. And the fact that an 18" barrel will likely require faster powders for a complete burn..... which may then promote cycling irregularities, is something else that we don't often hear discussed. Unfortunately, the world of AR15 websights/forums is waaaay too full of vague/simple answers that I refer to as "The dreaded one liners" . (ie; folks that post nothing much more then a "yes", "no" or "I did one of those, and it will work") Evidently, there's only a fraction of really knowlegable smiths or hobbiests out there who know the ins and outs of an AR compared to the field of bolt actions.....and are willing to share it.


Thanks again. This will be my 3rd 308 AR barrel. If I ever get the new build to fire thos 5 shot 5/8" groups as the norm, I'll post the photos here to you in your honor.
 
VaniB

I'm mystified as to why you pose a question and then answer it yourself by quoting a response I made on a different thread to a different guy and to a different question almost a year ago. I'm further mystified as to why you feel compelled to critique my post as being insufficient for you when it wasn't a response to you or to a question you posed. How about not quoting me out of context from now on. Do me another favor as well. Put me on your ignore list with alinwa. I do enjoy his company.

Sincerely,

Greg
 
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VaniB said:
Thank you Mr. W for such an in depth reply. When folks post these kind of technical stats, they often don't take the time or effort to explain/translate how it physically works in the chamber. Your input was very helpful to me so that I now feel confident that this is the chamber I should seek for my next barrel to be made by Krieger.
Click to expand...


Ummm....
 
Greg Culpepper said:
VaniB

I'm mystified as to why you pose a question and then answer it yourself by quoting a response I made on a different thread to a different guy and to a different question almost a year ago. I'm further mystified as to why you feel compelled to critique my post as being insufficient for you when it wasn't a response to you or to a question you posed. How about not quoting me out of context from now on. Do me another favor as well. Put me on your ignore list with alinwa. I do enjoy his company.

Sincerely,

Greg
Click to expand...




Mr. Culpepper,

I'm sorry for this misunderstanding that you have.

It was simply that my thread was going unanswered for too long, and so I took the initiative to check into the archives. In the archives I did find your old post which has answered my question here about the Obermeyer chamber. While your archived input is concise, it is still also very informative and describes the chamber very well. I merely paraquoted your old post for the purpose of describing the Obermyer chamber to anybody else that might also have liked to read and learn a good reply to my question. I'm sorry that you read anything else into it. When I stated to Robert that I was so tired of the one liners, and that it was kind of answers like "yes", "no" or "I did one of those. It will work." as an exact example of what I was referring to. (and what I'd often experience at the "weekend warrior" sights.) I can see how you might have misconstrued my "one liner" comment as being directed to you, but it was not.

Gosh....you must have done a lot of checking up on me in the archives to see who is on my ignore list. Even though it's more of a personal thing between me and that individual and it doesn't involve anybody else, you did bring it up, and I feel like perhaps you think I singled this man out for no good reason; I regard this individual as indeed very experienced in this sport. But it doesn't give him or anybody else the right to be needlessly hurtful and rude; I had just completed a 9 month long project, for which I was very happy with the finish, and provided photos of the rifle. This individual went out of his way to tell me that my tru-oil gloss finish looked like sh*t, and that's just the way he saw and called it, and I shouldn't expect any compliments here at BR Central if they weren't warranted. (to pretty much paraphrase what he said) I've gotten many compliments on the forums and at the firing range for this 22x47L rifle's finish.....but the point is if it wasn't his taste and he didn't like it, he could have just refrained from making insulting and hurtful remarks. We don't have to tell our neighbor that his wife reminds you of a Great Dane....even if we feel it's the truth.

I consider you one of the more advanced members of this sight and appreciate your input. I realize your harsh reply to me was only a justified response to what looked like an attack on you. Again, I aplogize for that misunderstanding, and hope there will be no hard feelings.
 
VaniB,

Thanks for taking time to explain. My reaction was to the combination of your quoting me out of context, Robert's complete and summary dismissal of my quoted comment (from memory it led off as "Unfortunately this is wrong", since edited out by Robert) and your snappy comment in return led me to infer a setup, at my expense, to provide Robert with a further opportunity grow his image as a preternaturally accomplished encyclopedic source of all things ballistic.

Glad to learn I was wrong. It can sure be tough saying what you mean on the internet.

Greg
 
First I am not an AR "expert" A few years ago I did make up three AR-10 in 308 with 18" barrels. Mostly they were a pain.

My very limited experience suggest that with the 18" barrel a mid length port has some advantages. I used the tube with two wraps around the barrel and the port mid length. It worked best with the faster powders. If done correct I found not accuracy loss from the brake. Problem is the with the 18" barrel there is a lot of pressure when the bullet exits = NOISE!. Faster powder helps on this slightly.

For me the 20" barrels seem to be a better over all choice.
 
How do the special forces guys get the 9" bbl to work with GI issue ammo????.,,,,these lil' guns are amazing...Roger
 
There seems to be

a feeling prevelent here that if someone asks a question, they should be completely ignorant of the subject and not discuss what they believe they know about it after someone answers them. I have seen this happen time and time again.

If we were in a group and someone asked a question and then stated what they thought they knew about it after the question was answered, most peeople I know would not be offended. Why is it offensive in writing and not in verbal in person conversation?

Just because someone asks a question it doesn't necessarily doesn't mean they are totally ignorant of the subject matter and does not mean they are bound to accept the anwers they are given or not allowed discussion. Most folks aren't looking for THE ANSWER MAN, the all knowing, all seeing ONE. I believe most are looking for differing opinions and discussion. If a person't can't handle that reality, perhaps they should not participate here, IMHO.
 
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