6BRX in an AR-15

[fa18hooker]

I've built a 6BRX upper for my AR-15, but as expected, I'm having cycling problems...to wit, failure to fully stroke. The rounds feed fine, fire fine, and extract fine, but then the bolt closes on an empty chamber. Best I can tell, the bolt isn't going far enough aft to pick up another round. I'm shooting 30.5 grains of Varget behind a 107 Sierra in a Shilen 1:8, using an oversized AR Performance .473 bolt and barrel extension. I've also installed an extended (2" further) gas tube. Not sure if I should lighten the buffer spring (currently standard GI), lighten the bolt, or change the load. Any help is appreciated.

 

[dieselten]

There are several things to consider:

- BE SURE you are actually short stroking. Put one round in mag and fire. Bolt should lock back. If not, you are short stroking. Also be sure bolt will lock back when manually operating the charge handle on empty mag. Sometimes the buffers are too long.

- check for gas leaks at gas block and the staked screws on the bolt carrier for the manifold tube. Also make sure the gas block is centered (front/rear & side/side) and that the hole in the gas block is centered over the hole in the barrel.

- your load seems light. I use 30.5 Varget/107 Sierra in my regular 6mmBR

- what buffer are you using? Try a lighter weight buffer. Do not change the spring, always use a standard carbine spring in a carbine buffer tube or a standard rifle spring in a rifle buffer tube. If using a rifle buffer tube you can use the lighter weight carbine buffers if you also use an extension. But this should not be necessary if everything else is OK.

- the gas port being 2 inches further forward reduces the amount & duration of gas getting back to the bolt.

- last option, enlarge gas port. Don't worry if you go too big, you can get an adjustable gas block to fix that. Just don't F-up the bore when drilling.

 

[rcw3]

My first advice is find out what's really wrong before you potentially wind up treating the symptom of a problem instead of the underlying problem (I see people do this all the time and just compound problems).

You have not given enough information to permit a meaningful reply on the condition you have with your upper:

1. Barrel length?

2. Port size of your gas port?

3. If you load one round in the magazine and load and shoot it does it cycle back so the bolt catch holds the carrier back (i.e. all the way back to the face of the bolt, since when an AR is close to short stroking it can also catch on the forward face of the carrier and give the impression it is cycling back far enough when it is not)?

4. What are you doing for magazines?

5. I would want to also see your chamber reamer design as that can play heavily into whether the rifle will cycle properly and normally (or not). I can't tell you how many reamer drawing designs I see that people use with AR's that are totally inappropriate for a semi auto gas gun AR-15 - they shove the chamber in there and then wonder why they have cycling problems and then go through all kinds of gyrations trying to solve what's wrong without ever knowing it's really their chamber design - and be careful - my experience is most reamer makers don't necessarily know off a wildcat cartridge what makes for a good AR-15 reamer design. The whole BRX reamer designs came straight out of bolt gun F-Class and Bench Rest shooting where they can get away with tolerances and dimensions that are a recipe for functional issues in an AR-15.


Some impressions right off -

- - Moving a port forward has pluses and minuses and is not always appropriate - without answers to the questions above I can offer no info on your use of that here.

- - I do not agree that it does not matter if you open the port too much because you can throttle it back with an adjustable gas block, as too big a port has other adverse consequences whether or not you throttle back excess gas.

- - A carrier weight is the reverse of what you might typically need if you are short stroking as it is typically a retrofit for an over ported rifle. I would work off a standard buffer tube, standard buffer and standard buffer spring and get it working with that as a baseline, then go from there if you want to tinker with after market stuff (if you are doing that at all).

- - The load is fine and it should cycle with that, and if it does not with that load, then you have other problems.

Robert Whitley
www.6mmAR.com

 

[fa18hooker]

Robert-
Sincerely appreciate your interest in my pet project...your advice is well regarded. To answer some questions: the barrel is finished at just over 26", and the gas port is .093 (.125 gas tube). When I have a single round in the C Products magazine, the bolt will not lock back after firing. Rounds will feed just fine when cycling manually (i.e. the entry angle is correct and the rounds don't hang up). As far as the reamer design, it's a Kiff 6BRX. Don't know if he makes different prints. I agree there's a problem if this won't cycle with 30.5 of Varget. I prepped some brass last night to load up this week in a ladder to see what it does. I also plan on putting a chrony on these rounds to see if that helps the diagnosis.

Thanks again,
Matt

 

[rcw3]

Robert-
Sincerely appreciate your interest in my pet project...your advice is well regarded. To answer some questions: the barrel is finished at just over 26", and the gas port is .093 (.125 gas tube). When I have a single round in the C Products magazine, the bolt will not lock back after firing. Rounds will feed just fine when cycling manually (i.e. the entry angle is correct and the rounds don't hang up). As far as the reamer design, it's a Kiff 6BRX. Don't know if he makes different prints. I agree there's a problem if this won't cycle with 30.5 of Varget. I prepped some brass last night to load up this week in a ladder to see what it does. I also plan on putting a chrony on these rounds to see if that helps the diagnosis.

Thanks again,
Matt

Matt

A 26" barrel length is fine for your set up. If you told me it was a 22" barrel with the port out 2" I would say that's probably an issue right away.

With the port an extra 2" out, with a 6mm, an .093" port is close to the edge function wise and I would go up to .096" without hesitation, especially since you are having sluggish cycling (.003" more does not seem like a lot but the extra gas volume is a fair amount more). We use an .096" port in some 6mm applications with the port out 2". If you just have to have more gas than an .096" delivers, then an .098" is o.k. too, but I would not go any bigger - if you would need to that is a clear sign you have other issues (maybe chamber design).

Most of Kiff's standard BRX reamer prints are really bolt gun prints and not quite ideal for AR use. Maybe some make them work in AR's but Kiff will make whatever you order up, so it's really ideal to optimize the design for an AR to start. Most of the PT&G BRX reamers have a .470" "hug the web" dimension .200" forward of the bolt face which is tight and not ideal for an AR (i.e. can make extraction sticky and slow down the cycling when a case is being extracted under pressure as in an AR). For an AR-15 I would use the standard 6BR .4714" dimension there. With an AR chamber a slight taper on the neck is good too and a little more generous neck clearance, and most of the BRX bolt gun reamers also do not have these features. At this point you have the chamber you do so there's not much you can do with that unless you specifically get a reamer to re-do some things.

The C - Products mag was made for the Olympic set up that has a much bigger diameter bolt and as far as I remember, those mags were designed to sit lower in the receiver. I don't know whether that's an issue or not with your bolt stop being pushed up sufficiently. If you put an empty mag in and pull back the carrier, does the bolt catch grab the face of the bolt and hold it back or is that marginal?

I hope this helps.

Robert Whitley

 

[philipmurphy]

Fa18,

I just built a .264 LBC with a 26" barrel and 14.5" gas system. I used a .102" gas port and a JP .935 adjustable gas block. Have not fired it yet as APGold fire control has not been delivered. I used a standard rifle buffer and spring. Heavy weight buffer and springs cause the BCG to batter the reciever. Am using a Young slick NM BCG, which is 1/2 to 1 oz heavier than stock AR15. C Products mags.

Intend to begin function testing with gas port closed, and open one turn per round until full cycle is achieved. I hope .102 is big enough.

Phil

 

[constructor]

Robert-
Sincerely appreciate your interest in my pet project...your advice is well regarded. To answer some questions: the barrel is finished at just over 26", and the gas port is .093 (.125 gas tube). When I have a single round in the C Products magazine, the bolt will not lock back after firing. Rounds will feed just fine when cycling manually (i.e. the entry angle is correct and the rounds don't hang up). As far as the reamer design, it's a Kiff 6BRX. Don't know if he makes different prints. I agree there's a problem if this won't cycle with 30.5 of Varget. I prepped some brass last night to load up this week in a ladder to see what it does. I also plan on putting a chrony on these rounds to see if that helps the diagnosis.

Thanks again,
Matt

Make sure the port in the gas block is aligned properly, you can open the hole in the gas block to cover the port in the barrel if not. Roberts right about port sizes on an extended length gas system, I try not to go over .100 but the 90s is the right range. I shoot a 22" but with a rifle gas system, I believe I have a .096 port. If you continue to have problems give me a call at the shop.
Also I have seen Mcfarland rings cause short stroking because they are so tight in the carrier, may just need a little break in.
H
BTW did you use Kiffs "6BRX-AR" reamer? If so that was the first BRX reamer he made for an AR about 6 mths ago, It works fine in my rifle but has a .100 leade so I have to load them out to 2.298 OAL.
.080 might be better for a 107.

 

[fa18hooker]

To follow up: hand cycled the bolt a thousand times with plenty of grease, and then took it out this morning to shoot it...tried loads of 27, 28, 29, 30 and 31 grains of Varget with magazine length 107's. Function just fine, so it appears the gun just needed to be "shot in". Glad to see the bolt/barrel extension combo worked as advertised. Still some minor problems with feed angle from the magazines (with the body taper of the 6brx, and the bullet noses are tending to point downwards in the magazine). I'm using the C Products 6.5 Grendel mags. After loading the mags are "thick"...they bulge in the middle so there's some resistance when inserting the mag, but it drops free just fine after firing. The first bullet also has to be pointed inward/upward slightly to make sure it feeds right and doesn't crunch the bullet on the face of the barrel extension. The last round (of 5) is also tending to hang up, but if I load 6 rounds, the 5 rounds feed fine. Not sure what the solution is...I'm going to polish the barrel extension "ramps", and maybe work on a new follower for the magazines. Hoping to use this for across the course highpower competition. Thanks to everyone for their advice...got me to check over all my work, and the solution was to just shoot some more!

 

[JZ1each]

Any of the loads showing promise?

 

[fa18hooker]

Didn't bench them yet...and forgot the chrony. The 30.5 load I shot out of the Punkin' Chunker bolt gun got 2850...and I'm sure I've got losses to the gas system, so I'm going to up it a little bit and see where it's at. The great thing about the AR performance bolt is there's minimal firing pin protrusion...no pierced primers like I had with the bolt gun.

 

[constructor]

if the follower is nose diving take a look at the spring. When held straight up if the only thing touching is the center hook of the spring bend the front loop just under the follower up so it supports the front of the follower like this.

 

[rminut]

any new news on your loading?? Has anybody identified a velocity node in the AR platform that seems to work well? I have a regular BR so I doubt i'll get the velocity you can but maybe I can get to a lower node.

 

[Klong]

rminut. With a 26" barrel try around 2780 fps with a 107 gr SMK. Somewhere within + or- 30 fps you should have a good load.

 

[constructor]

I have built 3 6BRXs(.271 neck, .100 leade) this last month all 22", all shot in the .3s with 33gr Varget, 95SMK set to 2.298(using PRI mags, apx .010 jump) CCI 450 primers. 105 Amax or 107 SMK@2.298, 35.3 RE17.
Had good luck with 34.8gr N550 also but there is none left in the country.

Waiting on a new 6BR reamer with a slightly shorter leade, other than shoulder location all other case dims will be the same as the BRX so it should be a good chamber.

 

[ericskennard]

Very intersting thread. I would have said to open the gas port in the barrel. I just built a 338 federal on a Remington 25 rifle. It behaved just like yours. It feed well, chamber fine, cycled but would close on an empty chamber. I found the gas port in the barrel was half as large as the port in the gas block. I opened it up and the rifle cycled correctly.

ERIC