AR15 Barrel Diameters
Is the outside diameter of a standard 7.62X39 AR barrel the same as a 5.56X45 standard barrel (not HBAR or Bull).
To err is human, to moo is bovine.
To err is human, to moo is bovine.
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Boeboe, I can't believe you're being ignored. I am at your service my friend. I put the mic to my preban 7.62X39 upper with a 16" barrel. When measured just ahead of the front sight the barrel diameter is .725". All of my preban AR-15 rifles are in Utah at this time but I've grabbed one of my restricted LE AR-15 rifles with a standard diameter 16" barrel. When measured in almost the same location (this rifle has the evil bayonet lug) the barrel diameter at this same point is an identical .725". Just as an experiment I've measured the barrel diameter of my COLT LE M4 Carbine and it measures a slightly larger .750". To continue the adventure; the heavy barrel fitted to my Colt light-machinegun upper measures a hefty .965". What else can I do tho assist you Boeboe? You don't ask questions for no reason...what are you up to??? Something interesting I am sure!
Mark T. Christian0 -
Thanks Mark, much appreciated. You may know I'm in the preliminary stages of building an AR pistol (from a receiver that's never been a rifle, or much of anything else for that matter). I've got some tooling I need on order, that will take about a month to get here. I've just been doing some preliminary work and calculations on the receiver, knowing the total weight of the pistol can't exceed 50 oz.
I have a Pro Ordnance Carbon 15 Type 97 pistol, I know there have been a lot of people who have had problem with them, but mine has been nothing but good to me. But to say that it's a blast to shoot is the absolute truth, the muzzle blast and flash are tremendous. Of course, this might be expected when shooting a rifle round out of a pistol. I have thought for some time that to optimize the performance of this type of pistol, it would be best to use as heavy a bullet as possible with fast burning powder, hopefully, that would reduce some of the blast.
In thinking about it further, it is really looking (to me) like a 7.62X39 configuration would make more sense in an AR type pistol than the 5.56X45. Several things point to that. The 7.62 caliber would seem more appropriate from the short barrel and in the range one would expect to use such a pistol in (not that I'm really sure any such pistol actually has any use at any range, but it would be pretty wild). Professional Ordnance was promoting a new AR rifle and pistol in 7.62X39, but of course, since they are in bankruptcy now, who knows if that will ever come to pass. Of course, they made just a few OA93's in 7.62X39, but I haven't heard of any of their new swiss cheesed OA98's in that caliber. So it looks like I am going to have to take matters in my own hands.
Why is the diameter of the barrel important? I'm sure you've figured it out, but for those who haven't, assuming the outside diameter of an AR15 7.62X39 barrel is the same as a 5.56 barrel, that would mean for any given length, the 7.62 barrel would be LIGHTER than the 5.56 barrel. Now, some of you might think that any weight difference between the barrels might be negligable. But from everything I've read, when trying to drive the weight of an AR pistol down below 50 oz, you literally have to cut as many corners as you can. Cut corners and drill lightning holes. So, it just seems to me the 7.62X39 makes (relatively) more sense in an AR pistol, and it's looking like that for a few reasons.
I am wanting as long a barrel as possible, again, for optimal preformance. For every gram of weight I save somewhere, it means the barrel can be a tiny bit longer. Of course, I need to take some margin of error into the weight as well, shooting for some 48 oz (which is the weight of the new Hesse AR pistol). The receiver I'm using is cast from ceramic matrix metal, it looks, weighs, and works like aluminum, but is about 40% stronger. That will allow me to make several adjustments to the weight of the lower without sacrificing any of the critical strength areas. And, I hate to say it, but the Hesse (bare) fiber composite upper receiver only make sense for this project.
So that's where I'm at right now. I know, your going to say the cost of the AR15 7.72X39 high cap mags is prohibitive, but I'm not planning on needing a bunch of them. This ain't going to be a primary weapons system or 'nuthin. Just a fun gun that's kinda interesting in a few ways. Two or three mags will probably do.
Oh, but don't tell anyone about this project, it's a secret!
To err is human, to moo is bovine.0 -
Boeboe, I was of course intrigued by your AR pistol plan. The idea of using a 7.62X39mm barrel is cleaver to say the least; the larger bore diameter will save a few fractions of an ounce. To help you further with your calorie counting, have you considered the difference in weight of the bolt carriers? Well my friend I am here to help you and I put three complete bolt assemblies on my Ohaus scale to see what each weighs. First off there is no measureable difference in weight between bolt heads and firing pins- perhaps a gram, but that is the extent. There is however a difference in the weight of the carriers.
A standard AR-15 semi auto bolt/carrier assembly weighes 10.85 ounces.
An earlier Colt Sporter bolt/carrier assembly (with the entire rear lower portion milled away to use in the "blocked" lower recievers weighes 10.60 ounces- a savings of a quater ounce! Not too much, but it can add up.
As a comparison, an M16 full auto bolt/carrier (banned for use in semi's) weighs a hefty 11.50 ounces- this is due to the extra metal present on the carrier's bottom portion.
What else can I do to assist you Boeboe? I am at your disposal.
Mark T. Christian0 -
Well...I am impressed with both the questions and answers! 101 0 -
You should consider using the lighter flattop upper receiver. The featherweight 16", 223 barrel weighs about 25 ounces less front sight. You can machine most of the barrel down to 9/16 dia including the barrel under the front sight and make up an aluminum gas block to fit. You haven't said what length barrel you have in mind but there are better weight reductions in trimming steel than aluminum. Also use an aluminum barrel retaining nut.
While the carrier could probably survive some lightening holes in the upper rear or equalizing both upper and lower milling cuts, the AR carrier & bolt assembly need all the mass they have to function with the feeble gas system. You could try it but figure on opening up the gas port. Someone makes an adjustable gas port so there is room for some experimentation.0 -
I appreciate all the good posts from everyone! v35, I'd like to get away with an aluminum flattop upper, I've been hearing that the Hesse fiber uppers have some pretty loose tolerances that can affect accuracy. I'm interested in any thoughts anyone has, yes, I did wonder how much I could take off the barrel over and above the standard dimensions. I've been wondering if flutting would help.
I've also been looking at the price of some of the used Carbon 15 pistols out there, it seems some of them are selling in the $400 to $500 range. It's occurred to me that buying one of these used, just for the parts, might be a good alternative to buying a parts kit. If the tolerances of a Carbon 15 upper would lend itself to better accuracy than a Hesse upper, maybe I should look at that. Again, I know there are lots of people who have had problems with the Carbon 15. I bought mine used, but in excellent condition for what seemed like a good price. I have suspected that the original owner had problems with it, sent it in for repairs, and then sold it as fast as he could just to get out from under it. I do know I've been very happy with it, and have been impressed with it's accuracy.
On barrel length, I really don't know how long I can get away with, but just want it as long as possible. As I accumulate parts for it, and do my lightning cuts, my strategy is this. I will weigh the upper and lower receiver parts as I go, less the barrel. When I get everything but the barrel as light as I can, then I will start cutting down the barrel incrementally, cutting the length down to the point that it would all add up to less than 50 oz. What that barrel length turns out to be, I'm not sure yet. I'd hope for at least 7". I know I don't want an AR pistol snubbie!
To err is human, to moo is bovine.0 -
boeboe, very interesting. Have you considered using aluminum parts that have a .003-.005 hardcoating applied to them. Coating can be applied as hard Rockwell hardness as you need. 0 -
The LTWT barrel is .562 dia forward of the gas block,.750 under the gas block and .625 aft of the gas block up to the front of the chamber. You could make the barrel
.562 dia up to the front of the chamber and taper it up to the original barrel dia at the barrel retaining nut. Reduce the barrel diameter under the gas block to about .570 (14.5mm) and make the gas block a light force fit over that.
Try your questions on WWW.AR15.com someone there may have already solved the problems.0
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