Understanding PCC- build questions

[dossauce]

Hey enosers,

Apologies if it's been asked, I searched but couldn't find what I was looking for. A lot of parts threads but no detail into the why's.

Like many of us I want to get into PCC but I would rather build one then buy and replace a bunch of parts. If I build one I would like to build it as an SBR but would like to use a standard ar lower so I can use it for a 5.56 sbr later on.

Can you attach a dedicated 9mm upper to a standard lower? I assume you will need a magwell insert which is fine.

I also see some 9mm uppers that look like they have universal ejection ports for 5.56 size cases, is there an advantage to this over 9mm uppers (besides changing the barrel, I mean performance wise, would one run better then the other)?

What's the deal with triggers for 9mm bolts? does it have to be mil-spec?

Do 9mm ar's use the regular 5.56 bolt? I see a lot of BCG's that don't come with bolts so my assumption is yes.

I won't even ask a question about buffers because it's been beat to death. I'll search once I get a better understanding of how its all working.

I assume PCC don't have gas tubes because of the fast burn rate of pistol powders? Ie not enough gas to push bolt back, therefore it uses the back pressure of the bullet to cycle bolt (meaning you need a heavier buffer/spring similar to slide of a pistol)?

I would love to see a sticky PCC thread that explains the difference between a standard AR in 223/5.56 and 9mm(PCC). It's a whole new world with alot of people making difference parts that don't play well with others, the opposite of what we're used to with AR's.

Thanks in advance guys and gals!

Edited July 18, 2016 by dossauce

[blaster113]

I just built my own 9mm AR using mostly CMMG parts and Brownells Colt pattern mags. Reliability was initially 100%, until I started tinkering with the buffer system... Don't do this, leave it alone once you build it.

-using a dedicated CMMG 9mm lower with a standard 5.56 upper. The ejection is straight out of the gun so no issues there. Small quibble, you can see smoke coming out of the gas port hole since the rifle is a blowback haha.

-a dedicated 9mm upper is nice but I don't think is necessary. I used a 5.56 upper I had in the safe to save money, but if I had to do it again I would buy the 9mm upper just cause it looks cleaner

-some 9mm AR bolts allow you to use standard AR hammers and FCG parts. The CMMG bolt in my gun allows me to use any mil-spec fire control parts. Other bolts require you to use a specially modified hammer

-the 9mm AR uses different bolts from the 5.56 version as its a blowback vs. direct gas impingement, thus no gas tube for the 9mm. the two bolts are not compatible

-definitely get a 9mm specific buffer instead of trying to make a std. carbine buffer work. the 9mm buffer is heavier than a regular carbine buffer to slow down the bolt

-My build was running 100% until I put a spacer in the buffer tube to restrict some of the bolt travel. now I had a few instances where the rifle would double. don't worry about putting a spacer in the buffer tube

-not to bash on anyone's product, but some of the cheaper makes don't seem to work right out of the box. We had a few guys try out their new guns at a match yesterday and almost all had issues. PM for details

[aandabooks]

I would build off of a dedicated lower. I bought mine from a guy on here. It is a PSA witha Glock lower. Swapped out the handguard, trigger and 6 position stock. Put on a JP comp. Runs great and has good accuracy.

[dossauce]

I know the Bolt Carrier is different but what about the actual bolt. Most that I see do not come with the bolt but I'm not sure if its because of how complicated the bolts are to make compared to everything else or what. I know even major AR manufactures rarely make their own bolts because of how complicated/ many cuts are required.

I have an idea, build it with a dedicated lower. When I build an sbr later on I can pick up a mag block and just move the upper over and see how it works. If it does then just shorten the barrel and I'm good to go on an SBR while using a carbine dedicated lower till I make that change.

[bwikel]

There is not a 2 piece bolt/carrier for 9mm blow back AR's. It is all one piece so what you are seeing is what you need and nothing more.

If you build a dedicated lower and want to run the upper on an AR15 lower with a conversion block you will need to go with a lower that uses colt pattern mags if you want to use the same mags for both lowers. There is currently no one that make an AR15 conversion block that utilizes glock mags.

[StealthyBlagga]

I have an AR15 SBR originally registered as a 7.5" 5.56NATO rifle. Now I run it in USPSA matches with a home-built 9mm upper; no-name forged receiver, CMMG ramped bolt, RRA 10" barrel, JP muzzle brake, midlength FF handguard, Aimpoint red dot optic. For a magazine block, I went with a Hahn top-insert unit, which is easy to install and remove - the only downside is lack of LRBHO (no great loss). FCG is a chopped standard GI M16 hammer and a JP trigger with standard GI hammer and trigger springs (not the yellow springs that come in the kit). Metalform 32-round mags have run 100%, and FMK-3 40-round mags are getting there.

I just built a similar setup for my son, but with a 16" New Frontier Armory barrel and a C-More optic... it has run 100% for him so far (much to my chagrin - he beat me yesterday)

Using a regular 9mm buffer lets me switch back and forth between 9mm and 5.56 at will, just by pulling the mag block and changing the upper.

Edited July 19, 2016 by StealthyBlagga

[dossauce]

Huh, interesting. Thanks Bwikel, i just assumed there would have to be a bolt.

Stealth, yeah I'm really leaning towards a New frontier side charging upper. Just trying to figure out what lower. I have glock mags but only 17rnd + TTI extensions so I'd be buying mags either way. Figured why not build an sbr lower and have at it.

Whats the deal with the trigger/ hammer combo? Do you need to use the GI hammer? Does the JP trigger reduce the pull if youre using GI hammer and spring?

[2MoreChains]

My understanding is if you get a ramped bolt you can use pretty much any trigger/hammer from mil spec to Gieselle to Hiperfire and probably a bunch of others in between.

This is the one I have: http://stores.9mmar.com/cmmg-ramped-9mm-bolt-assembly/

[dossauce]

Nice! and that shows the extractor. Thanks!. How has it been running for you? What trigger are you using?

[wgj3]

If you haven't figured it out yet, you will also need a heavier that standard AR15 buffer as well. I'm really enjoying the JP SCSS in mine. It's affords some ability to tune (change weights and springs) if you are inclined to do so. There is no "gas system" to turn up or down, so the spring and buffer are all that there is to adjust. The 9mm BCG is hefty compared to a standard small-frame AR BCG, and this extra mass is what is so rough on LRBHO designs. That monster carrier slamming into the bolt catch does nasty things. And, in reality, the feature should be needed seldom-to-never in USPSA competition.

Oh, and have fun! It's a blast once you get it going.

My 2 cents, Glock mags are much more prolific and have fantastic after-market support if so desired. I chose this route. However, the reload with this mag-style in an AR is not as natural. I think that it will become much smoother with practice, but its not the same as a pistol or a standard AR mag.

[dossauce]

Yeah i've noticed mag changes are rough. I wonder why pistols dont have that same issue with last round. in theory it should be the same amount of energy to cycle. Slide weight vs bolt weight. I wonder what the differences are.

[Chip3]

Here is a good topic on triggers. http://www.brianenos.com/forums/index.php?showtopic=235656As mentioned above, a ramped bolt is a good option.

Picking your mag system may be a good place to start. This will determine your lower and bolt. Glock mags have the advantage of crossing over to your pistol, have aftermarket base pads and are of good quality. Colts are double stack at the lips and in my opinion load easier and I like the straight in path when loading compared to the angle of the glock. Colt mags are not as wide spread and quality can vary (there are some bad glock clones out there as well)

As you gather your parts pay close attention to the specs of what you are buying. Especially if you mix from different manufacturers. There are not as many across the board specs like with a regular AR. Bolts can be colt cut , glock cut or both. Ramped or unramped which can determine your hammer choice. There are also some differences in weight between manufacturers. Barrel muzzles can be 1/2-36 threaded or standard ar threaded. Colts use a different LRBO than glocks. A free float rail may be the easier way to go as your barrel will most likely not have an FSB and there is no need for a gas block. If you use a carbine buffer instead of a 9mm specific you may need a bumper/spacer for the end of the buffer tube. Some use a stack of quarters.

Just completed a build based off a PSA dedicated colt lower. Prebuilt lower was less than the price of a block so made sense to me. It was put together well and looks good. The upper is a mix of parts and the only issue when assembling was having to file the ejector slightly to clear the bolt. It is not a PSA bolt. Have only put 100 rnds through it and 1st 50 of brass ran well. Next 50 of steel had some hangs, but feel that is more a result of $11 ASC 32rnd mags. Plan to try some metal forms next.

Some recommend KNS pins for the trigger with the thought being that the heavier bolt and blow back operation place stress on the pins.They may be worthwhile, especially if paying for a tax stamp. I used an Anderson sport upper and have found that lube and cleaner leak out under the hand guard through the gas tube port. May plug it, but something to remember when hosing out the upper.

[bishop414]

Wondering about a stack of quarters with a 9mm buffer? Any benefit or more risk of malfunctions?

[blaster113]

Wondering about a stack of quarters with a 9mm buffer? Any benefit or more risk of malfunctions?

I'd like a definitive answer to this as well. The spacers supposedly slow down the bolt to prevent breaking the bolt latch. My build was running 100% till I tried $1.75 in quarters in my buffer tube. Gun started doubling when my boys (age 10 and 8) shot it. I couldn't duplicate the malfunction. Took out 3 quarters and tested it; gun runs perfect. Friend of mine tried the $1.75 quarter trick and his new gun spat out a couple of 4-5 round bursts during a match.

Going to do more testing. Feel like leaving a spare bolt latch in my range bag and say screw it already. new bolt latch is $15

[Chip3]

If you have a 9mm length buffer you probably do not need the spacer. If you use a standard carbine buffer the spacer is supposed to help. The carbine buffer is lighter and shorter so the bolt is able to travel farther back resulting in faster motion forward. I would guess the gun above was doubling because the bolt was not traveling far enough back to reset the hammer and the hammer was riding the bolt back forward. The NFA bumper looks like a much better idea than a stack of quarters if you try to run a carbine buffer. http://www.joeboboutfitters.com/New_Frontier_Armory_9MM_40S_W_Buffer_Spacer_p/nfa-9mmbufferspacer.htm. With 9mm ARs being a blow back system and various bolt weights, buffer weights and lengths, 9mm loads, and buffer springs available you probably need to look at those items if you piece a gun together. My lower came with a 9mm buffer and seems to run fine.

[BigBamBoo]

I have put together a few 9mm AR's. My advice....use good parts from the start. The most important part is the buffer setup. Fr get spacers, quarters, etc.

Go with either Slash's (Heavy Buffers.com) 9mm buffer or JP's SCS.

To avoide bolt catch breakage, check over travel of bolt. Mine has about 1/8" at the most from the bolt face to the catch. You can also use a Seekins machined bolt catch as it is much stronger then the mil-spec ones.

Another thing I do to keep the bolt catch healthy is remove some material from the mag followers to allow them to engage the bolt catch all the way.

If you pull the charging handle back fast on an empty mag you might notice that the bolt catch is only half way "up". A way to check this is one you have the bolt locked back on a empty mag, is press down on the bottom of the bolt catch. I am willing to bet it will move inward moving the bolt catch "up".

After a few hundred/thousand rounds, the bolt catch gets deformed on the inner top part. By "tuning" the followers to allowed them to travel further up the mag body, they will fully engage the bolt stop as it is meant to.

9mm AR's are really fun to shoot. But they have their own set of issues to deal with.

[dossauce]

Agree with alot of points. I see no reason to try carbine buffers/ quarters. I'll build it right the first time.

[blaster113]

If you have a 9mm length buffer you probably do not need the spacer. If you use a standard carbine buffer the spacer is supposed to help. The carbine buffer is lighter and shorter so the bolt is able to travel farther back resulting in faster motion forward. I would guess the gun above was doubling because the bolt was not traveling far enough back to reset the hammer and the hammer was riding the bolt back forward. The NFA bumper looks like a much better idea than a stack of quarters if you try to run a carbine buffer. http://www.joeboboutfitters.com/New_Frontier_Armory_9MM_40S_W_Buffer_Spacer_p/nfa-9mmbufferspacer.htm. With 9mm ARs being a blow back system and various bolt weights, buffer weights and lengths, 9mm loads, and buffer springs available you probably need to look at those items if you piece a gun together. My lower came with a 9mm buffer and seems to run fine.

I'm running a CMMG 9mm bolt, CMMG 9mm buffer and a standard carbine spring. Also using an ALG Quality Mil-spec trigger with standard springs. Pull weight is about 6 lbs, feels a little more crisp than the CMMG FCG parts.

A 9mm AR is such a blast to shoot. Also much easier to reload for vs. a .223. My 9mm build is my favorite gun

[MarkCO]

With a carbine buffer tube, 9mm buffer and a JP bolt, the travel was still too long for my liking, over an inch past the bolt catch. I cut about 5/8" off a polymer 9mm spacer (from Brownells) and now have 3/8" travel past the bolt catch which is about right.

QC10 Glock mag lower, POF trigger, Seekins safety, AR15 upper with a plugged FA, Faxon 16" melonite barrel, 9mm pistol comp and a 15" HG. Topped with a Burris ARF3 optic.

[gnhowell1]

Wondering about a stack of quarters with a 9mm buffer? Any benefit or more risk of malfunctions?

I'd like a definitive answer to this as well. The spacers supposedly slow down the bolt to prevent breaking the bolt latch. My build was running 100% till I tried $1.75 in quarters in my buffer tube. Gun started doubling when my boys (age 10 and 8) shot it. I couldn't duplicate the malfunction. Took out 3 quarters and tested it; gun runs perfect. Friend of mine tried the $1.75 quarter trick and his new gun spat out a couple of 4-5 round bursts during a match.

Going to do more testing. Feel like leaving a spare bolt latch in my range bag and say screw it already. new bolt latch is $15

Sounds like bump firing the gun from not holding it tight.

[bwikel]

My guess is it would be caused less by bump fire and more by the hammer not resetting and following the bcg as it goes back into battery. If you used a 9mm buffer and a spacer you effectively do not let the bolt travel the full cycle to reset the hammer properly.

[gnhowell1]

My guess is it would be caused less by bump fire and more by the hammer not resetting and following the bcg as it goes back into battery. If you used a 9mm buffer and a spacer you effectively do not let the bolt travel the full cycle to reset the hammer properly.

Even though he couldn't duplicate the malfunction his kids had?

[rishii]

With a carbine buffer tube, 9mm buffer and a JP bolt, the travel was still too long for my liking, over an inch past the bolt catch. I cut about 5/8" off a polymer 9mm spacer (from Brownells) and now have 3/8" travel past the bolt catch which is about right.

QC10 Glock mag lower, POF trigger, Seekins safety, AR15 upper with a plugged FA, Faxon 16" melonite barrel, 9mm pistol comp and a 15" HG. Topped with a Burris ARF3 optic.

Is this the spacer

http://www.brownells.com/rifle-parts/receiver-parts/buffer-tube-parts/buffer-tube-hardware/ar-15-m16-fixed-stock-to-carbine-spacer-prod44739.aspx?avs%7cMake_3=AR-15

[MarkCO]

With a carbine buffer tube, 9mm buffer and a JP bolt, the travel was still too long for my liking, over an inch past the bolt catch. I cut about 5/8" off a polymer 9mm spacer (from Brownells) and now have 3/8" travel past the bolt catch which is about right.

QC10 Glock mag lower, POF trigger, Seekins safety, AR15 upper with a plugged FA, Faxon 16" melonite barrel, 9mm pistol comp and a 15" HG. Topped with a Burris ARF3 optic.

Is this the spacer

http://www.brownells.com/rifle-parts/receiver-parts/buffer-tube-parts/buffer-tube-hardware/ar-15-m16-fixed-stock-to-carbine-spacer-prod44739.aspx?avs%7cMake_3=AR-15

Yes. I just cut a short piece off of that. You need to make sure that the trigger re-sets and you have at least 1/4" of travel past where the follower will engage the bolt stop. A little more does not hurt. I have about 1" of travel past where my trigger re-sets and 3/8" past where the bolt stop engages. Just realize doing it "dry" may have different results than when actually shooting, so be prepared to test it out a bit and possibly make adjustments.

[rishii]

Thank you for the info