Pistol Caliber Carbine

[ranger]

For Colt style 9mm mags I am using my old Safariland G21 mag pouches

[rd62]

I'm in the process of building a Quarter Circle 10 9mm AR pistol using one of their Glock Small Frame lowers to utilize the plethora of 9mm Glock mags I already have.

I need to pickup a buffer and spring to complete the lower and wondered if the collective had any suggestions as this is my first 9mm AR? I'm leaning toward the one Slash at Heavy Buffers makes.

[SeriousStudent]

You'll want a Colt 9mm buffer, and a Colt 9mm Receiver Extension Spacer.

Brownell's is currently out of both. I use both of those in mine, and they work very well - especially with the suppressed subsonic ammo, if that is your goal.

[rd62]

You'll want a Colt 9mm buffer, and a Colt 9mm Receiver Extension Spacer.

Brownell's is currently out of both. I use both of those in mine, and they work very well - especially with the suppressed subsonic ammo, if that is your goal.

Thanks. Yours is a Quarter Circle 10?

[SeriousStudent]

Mine is a New Frontier, that was rebranded for these folks: http://www.runnerrunnerguns.com/prod...r-billet-w.htm

A buddy has a Quarter Circle 10. We swapped out my buffer, spacer and spring for his. He was having constant malfunctions using a H2 buffer, regardless of magazines or ammo.

I dropped my spring, spacer and buffer into his, and he was able to run with no malfunctions. Magazines included the factory Glock 33-rounders, as well as the ETS and Magpul mags. Ammo was Speer 147-grain subsonics, along with Winchester 124-grain NATO, as as IMI 158-grain subsonics.

Hope that helps.

[mmc45414]

You'll want a Colt 9mm Receiver Extension Spacer.

As an expedient for the spacer you can drop a stack of quarters in the tube, I think the magic number is five. I keep meaning to buy the spacer but the thing keeps running, I had forgotten all about it.


Sent from my SAMSUNG-SM-G870A using Tapatalk

[DpdG]

Mine is a New Frontier, that was rebranded for these folks: http://www.runnerrunnerguns.com/prod...r-billet-w.htm

A buddy has a Quarter Circle 10. We swapped out my buffer, spacer and spring for his. He was having constant malfunctions using a H2 buffer, regardless of magazines or ammo.

I dropped my spring, spacer and buffer into his, and he was able to run with no malfunctions. Magazines included the factory Glock 33-rounders, as well as the ETS and Magpul mags. Ammo was Speer 147-grain subsonics, along with Winchester 124-grain NATO, as as IMI 158-grain subsonics.

Hope that helps.

I also have a NFA/Runner runner complete lower, with a complete PSA upper. I originally used the included CMMG 9mm buffer (5.3oz), spring, and spacer, which worked. Having said that, it ejected violently (brass went 20+ feet) and you could feel and hear the buffer slamming into the spacer/buffer tube. I ended up getting an extended heavy buffer from Macon Armory made up. For some reason 7.8 ounce stands out in my mind, but it's an adjustable weight using lead shot. The new buffer, using the original spring without a spacer, greatly decreased the violence of ejection, although it is still very energetic.

-G

[SeriousStudent]

Interesting, thank you for the info. I'll try and that pass it onto my friend as well.

I've got a Griffin Optimus in the NFA pipeline for mine. It's going to be a dedicated can for that firearm. I've got a Silencerco Omega 9K in process for my CZ Scorpion.

[fishing]

I'm in the process of building a Quarter Circle 10 9mm AR pistol using one of their Glock Small Frame lowers to utilize the plethora of 9mm Glock mags I already have.

I need to pickup a buffer and spring to complete the lower and wondered if the collective had any suggestions as this is my first 9mm AR? I'm leaning toward the one Slash at Heavy Buffers makes.

You'll want a Colt 9mm buffer, and a Colt 9mm Receiver Extension Spacer.

Brownell's is currently out of both. I use both of those in mine, and they work very well - especially with the suppressed subsonic ammo, if that is your goal.

Overly verbose and detailed replying warning .

rd62 -

I'll offer my $0.02 on this, an opinion that differs slightly from SeriousStudent's (I'm not discounting his experience and advice completely as he stated it works well, but I do not believe it to be the optimal setup based on my experiences).

I've been 9MM PCC crazy for a few years now, partially because they are quite fun IMO and also because I enjoy having different, quirky and unique stuff (I guess I am hipsterish in that way!).

I will refer to blowback action, AR type 9MM PCCs as the “AR9”, although it may not be the completely correct nomenclature.

At this point, I’ve been building AR9s for over 5 years. Much of what I know about getting them running right is based on empirical learning and costly trial/error. It was rather annoying to build one a while back due to the lack of component manufacturers, but its great to see that for the past year or two PCCs have really taken off and there are many options available.

Related to your question -

• Much as with the 5.56 AR, I believe having as heavy a bolt/buffer as possible before going overboard and negatively affecting cycling is the proper way to go. Lightening recip mass on a comp gun with a locked bolt can work well when tuned properly, but is not the way to go for straight blowback.
  
• I have tried but not objectively found any benefit to reciprocating mass inside the buffer for an AR9 - either solid or reciprocating are fine with me. I think reciprocation is important for a locked breech design as found in the AR15 though - and I have not found any reason to pursue one of the various hydraulic buffers on the market for the AR9.
  
• According to such reliable sources as the internet, optimal bolt weight for a straight blowback 9MM is 1.7#. For reference a closed bolt semi, full size Uzi bolt weighs ~23 ounces. I treat the combined weight of the AR9 bolt and buffer as one, so at a minimum the two together should have a mass of 1.5# and function is 100% at the current combined setup I run in my AR9s which is ~26.5 ounces (close to 1.7#).
  
• Bolt travel distance is of paramount importance to the proper functioning of the AR9. Bolt length is limited by the length of the upper receiver, so the shorter length of travel must be made up by the combination of the buffer tube, buffer, spring and any spacers used.
  
• An essential “test” after building an AR9 but before firing is what I’ll term a “travel distance check”. Assemble the gun, then load an empty mag and lock the bolt back. Then while looking through the ejection port, pull the charging handle back so that the bolt goes back as far as possible and take note of how far this is from the bolt stop lever in the lower. If not already, add spacers or change buffer/spring so that the distance between bolt and bolt stop lever is less than ~3/8th of an inch.
  
• Test fire with to ensure gun is running reliably. Add a quarter if the gun runs reliably, if not remove one and check again. I’ve ended up at just ~¼ inch for my guns. There is some variation in spring/buffer tube/buffer/spacer/lower combos being used so this tweaking ensures that the bolt travels rearwards during cycling “just enough” before coming to a stop and bouncing back due to spring action. Excessive rearward travel means higher bolt speed and cycling issues as well as battering of the bolt stop. I’ve broken a number of bolt stops and ruined an upper before figuring this out.
  
• Quarters make good spacers - simply drop them into the buffer tube being sure that they lie flat when they reach the rear end of the tube. I do not know enough about spring dynamics/behavior in the buffer tube to know of the effects of taking up space this way (compressing the entire spring) versus the use of some of the commercially available spacers that take up space within the spring while only slightly affecting spring length. I wish I could ask some of the folks at Geissele or KAC etc to see if they would have enginerdinging thoughts on this.
  
• Buffer spring choice itself does not seem to have huge effect on AR9 function, so long as the spring/bolt/spacer combo used passes the “travel distance check” mentioned above. My understanding is that spring strength has negligible impact on the rearward portion of the operating cycle of a blowback gun but is more important on the forward portion of the cycle (feeding). Due to this, I use AR10 Carbine length buffer springs - they cost the same and have a higher spring rate than AR15 carbine springs.
  
• The reviews for the colt buffer (I’ve never used one) seem to indicate that it is on the lighter end, and may not be as long as optimal. Fuck paying $35 for a buffer without tungsten in it.
  
• My recommendation to you would be to get an AR10 carbine length buffer spring, and either of these two buffers depending on the weight of whatever bolt you are using. Both happen to be reciprocating and are long enough to need minimal # of quarters
  
  https://www.kakindustry.com/ar15-9mm-buffer-long - 8 OZ and $32.
  https://www.kakindustry.com/ar15-9mm-buffer-long-heavy - 10 OZ and $50.

Now the below which may be useful in the broader context of the thread -

• AR9s can be very finicky to get running 100%, chances of consistent reliable function are better with high-quality factory built complete guns as well as guns built using known good/common mfg components for critical items (bolt/barrel/lower etc).
  
• For the purposes of the type of competition described in this thread, the current MPX is the best 9MM PCC IMO. I am not completely satisfied with the two I own though - and WAS/AM FUCKING PISSED about the fiasco that was my first gen MPX and the ongoing issue of the caliber conversion kits etc. (Shame on me for being an early adopter as well as believing anything info SIG put out, but thats a separate thread).
  
• MP5 type guns have their merits but are mostly just historically interesting to me at this point. CZ Scorp is perfectly fine but doesnt rustle my jimmies in any special way.
  
• AR9s run dirty as hell - some barrels are cut with quite generous chambers such that even when tuned right, brass will come out sooty as hell.
  
• QC10 makes the best "AR9" lowers currently IMO/IME due to them (and DDLES by extension) making the original dedicated form factor magwell AR9 lowers, retaining LRBHO etc etc. I’ve been very happy with mine in terms of quality, function, looks, fit and finish as well as CS.
  
• Side charging uppers are especially great on AR9s - I mostly use the Gibbz type upper on my AR9 builds (my understanding is Gibbz mfgs the uppers and sells them as well as supplies them for others to brand/sell which is good as I dislike the GIbbz logo on them). Gas blowback to face is noticeably reduced using one one when I run suppressed.
  
• Chamber face geometry, ejector tuning and magazines are critical to reliable cycling of AR9s - majority of related issues are from folks who have built them without making sure these components play nice with eachother.
  
• AR9 recoil impulse is unpleasantly harsh, and they have more felt recoil force than your average 5.56 AR, but in the grand scheme of things it’s not bad by any means.
  
• AR9s are harsh on components especially when not built optimally, fun as hell to occasionally run a magwell adapter on one of my FA lowers but I genuinely have concerns about stressing/fatiguing the lower with overuse.
  
• I believe that the creation of the AR9 was an afterthought that happened to work out OK - I would love to have a full understanding of the circumstances of the genesis and development of the gun back at Colt back in the 80’s.
  
• I don’t really see the utility of AR15 9MM magwell adapters nowadays, with the good dedicated lower options currently available other than use in a full auto lower. I suppose one could save some money going that route vs. dedicated lower but it seems like a hassle so for the gent who’s looking to build an AR9, hands down get a dedicated lower.
  
• I have experienced freaky unexpected accuracy (MOA out to 100 yards) from the AR9s I’ve built, no idea exactly why. Its also fun to be able to consistently ring man sized steel at 400 yards with one, hearing a dull thud over a second after firing. (just for shits and giggles of course, and only on windless days).
  
• My first QC10 lower was a colt mag pattern one, I made the decision based purely on cosmetics, even though I have a plethora of glock mags. Now after owning a couple of glock mag lowers as well, I still believe that colt pattern > glock pattern. They are easier to load than the single feed glock mags IMO too.
  
• I can drop and reload my colt pattern guns measurably quicker than the glock ones, IMO due to the straight up insertion.
  
• Metalform colt mags (including the PSA branded ones that occasionally go on sale) > all others in terms of quality and reliable function.
  
• No reason to not get a glock cut AR9 bolt IMO. No idea why MFG’s dont reduce a SKU and only make them versus offering both colt and glock cut bolts at this point.

On a related finishing note, myself and an C2 SOT buddy who is a skilled machinist are playing around with a couple of related fun projects - a fluted chamber barrel for AR9 we cooked up that is producing some interesting results, and we’re awaiting BATF opinion on a prototype gun we developed that allows for advanced primer ignition while in our understanding does not afoul of the ban on new manufacturer of open bolt semi-autos. Should be interesting.

Wish we had the computing power and software available to model this stuff, but since we arent a huge OEM, empirical tinkering will do for now.

[rd62]

So the Quarter Circle 10 bolt weights 15.1oz so should I go with the 8oz for 23oz or the 10oz for 25oz?