9mm AR Heavy vs. Light operating system. Share your experiences.

[Patrick Scott]

I built my 9mm AR about a year before we(USPSA) started talking about the division. As a PRD shooter I thought it would be neat to have an AR that shared all the ammo I was already loading.  All of my research produced info that said heavy(7-10oz) buffers and heavy(308) springs would give the best felt recoil. I never looked for any info on what kind of sight movement that would produce. I started the gun with a JP 308 recoil spring and colt 5.5oz 9mm buffer. Recoil impulse was smooth, I liked it when shooting my 130pf PRD ammo, Chrono tests showed my PRD ammo would run mid to upper 140s in PF. Down the road I bought a 7oz Hahn sub-cal buffer that smoothed the gun out even more. Yeah the dot would pull off of target when the system bottomed out and when the bolt closed, but who cares. Im just plinking.

Fast forward to PCC happening in USPSA.... 

Now I want as little dot movement as possible with no care how the gun feels. I talked to a few other PCCers and was convinced I should try a lighter operating system and load for the gun.  I loaded up some ammo that would produce an even 130pf out of my 16" barrel(PRD load -.2grs) and grabbed a 3 different buffers, two different springs, and headed to the range to test their effects on dot movement alone. Buffers were 1) 4.6oz H2 standard carbine buffer 2) 5.5oz Colt 9mm buffer 3)7oz Hahn sub-cal extended buffer. Springs were 1) Standard 223/556 carbine 2)JP .308 carbine  

Trying all buffer and spring combos, running strings of rapid fire on an 8" plate at 12yds I found that the using the H2 buffer and 223/556 spring gave the least amount of dot movement both when the system bottomed out on opening and when the bolt closed up. The Hahn buffer and 308 spring gave the most dot movement.  Yes the lighter stuff doesnt "feel" as good as the heavier stuff, but I'll give up feeling for reduced dot movement any day.  

My thoughts, the heavier setups are probably OK for the guys/girls going out and plinking with higher PF ammo, like factory and defensive stuff. Using hotter ammo, the heavy system may save some wear and tear on the gun by smoothing it out. For USPSA shooters, running minor and/or loading to their specific gun I feel the lighter systems might be the way to go. 

On a side note: While running the heavier system and a light trigger(Elftmann and JP EZ roller kit) I would occasionally get on the shoulder bump fires when trying to shoot fast or from awkward positions. So far this has not happened in light system testing. Makes me wonder if all the weight from the heavy system played a role in that when it all came forward to a stop??

Side note #2: FWIW I am running a JP bolt designed for the Silent Capture System. That means it has a through hole in the body to take the guide rod of the SCS. JP tells me this bolt is about 2oz lighter than normal off the shelf 9mm bolts.

To all you guys/girls that have never tried the lighter stuff, I suggest you give it a shot. The results might please you.

 

 

 

Or maybe Im crazy

 

 

 

Edited October 5, 2016 by Patrick Scott

[Dannybot]

Some of the 9mm AR makers recommend against anything other than milspec triggers.... 

 

Thanks for the write up.  

[Patrick Scott]

7 minutes ago, Dannybot said:

Some of the 9mm AR makers recommend against anything other than milspec triggers.... 

 

Thanks for the write up.  

At this point I am using the JP EZ Roller kit. It is the same setup JP uses in the GMR 9mm AR, I am also using their bolt for max compatibility.

[MikeRush]

You are not crazy! I've gone as low as a 3 oz buffer with my CMMG bolt and found less dot movement and quicker splits. It did exacerbate a feeding issue I had but I ordered a 4.6 oz H2 buffer for it a while back, and might even swap out one tungsten weight for a steel one to get to "H" 3.8 oz weight. That may be a good place to be for true minor loads.

I will say that WWB (chronos over 150 in my PCC) was downright nasty with the standard 3 oz carbine buffer. Lots of noise and gas from the ejection port.

Have you experimented with bolt stroke at the different weights? I think I like the light buffer with full stroke best, but shortening the stroke does seem to speed things up a bit at the expense of some additional harshness as the buffer bottoms out.

[Patrick Scott]

Right now I have a standard car buffer and a H2 built, and some spare steel and aluminum weights kicking around. I may take them apart to test some middle ground weights, but I really want to keep the H2 intact so I can do back to back comparisons with having to build buffers at the range.  I may have a friend that will let me work with some of his stuff to solve that.

WWB was pretty nasty out of my gun even with the 308 spring and 7oz buffer which normal makes the gun feel softer. Non PCC guys trying out the gun were amazed at the difference in the gun when swapping between my PRD loads and WWB. 

I have messed a little with stroke length. The 7oz buffer is extended to the point the when it bottoms out the bolt just barely clears the bolt catch. Which is really what you want in a 9mm to prevent the operating system gathering a ton of momentum before hitting it on a bolt hold open equipped gun. Thats what breaks bolt catches.  Mine doesnt have bolt hold open on empty so its never been a concern of mine. Since that buffer is extended I was never able to mess with stroke length on that one. I tried a few different stacks of quarters in the end of the buffer tube behind the 5.5oz and found that while the gun cycled faster, it felt harsher, and was easier to bump fire from the hip  at that point I was still plinking with the gun so I cant speak to dot movement. I do like the feel of lightweight/full stroke. Pretty much just like you. Just for the sake of trying, Ill mess with stoke length on the light system at some point. 

[Les Snyder]

after the 250 round PCC match last Sunday, I noticed some battering of the barrel breech face with my heavy 7 1/2 oz buffer and .308 spring... so changed my thinking after remembering your comments in the original post ... thank you .. I shoot a rifle length system as I like the feel of an A1 stock, so dropped back to a standard 5.2oz rifle buffer with rifle spring.... I tested it today with a rifle buffer of H2 weight (4.2oz made by removing two steel weights)... my particular carbine had less scope movement with the 5.2oz buffer... I really didn't want to load two different 9mm cartridges, but my 130pf load for the G34 is still  a little hot for the carbine...

 

side note... Mike Foley (USPSA President) was at the match and is seeking comments from PCC shooters... he is supportive of the new division

[Aircooled6racer]

Hello: Since you guys have been playing around longer with buffer and spring setups what are you guys using now for loads in the 130-135PF range? Thanks, Eric

[rowdyb]

iirc, when i had a dedicated cmmg 9mm ar rifle i switched to a standard 556 buffer and a rifle spring in a carbine length tube. then i went to a spikes tactical 9mm specific buffer with a rifle spring in carbine tube and was pretty happy about that.

yes, i seemed to break about 2 bolt catches per year but i was happy with the shooting and sight movement. mike is in a club that's really embraced pcc and it's cool to see. i loved shooting the above mentioned gun in LDF's steel match back in the day when that was the only place to shoot a pcc.

[Patrick Scott]

4 hours ago, Aircooled6racer said:

Hello: Since you guys have been playing around longer with buffer and spring setups what are you guys using now for loads in the 130-135PF range? Thanks, Eric

Im still running a H2(4.6oz) buffer and carbine spring behind a JP bolt. Soon as I get some time I plan on trying some slightly lighter buffers and pick up a JP SCS to mess with.  KAK Industry makes a neat buffer kit where you can mix and match to build a buffer you like. Under 50 bucks and gives you tons of options. Since I have so many AR buffers kicking around I like to take what ever happens to be my current favorite, then build different combos out of the KAK kit to compare side by side. 
 

https://www.kakindustry.com/ar-15-parts/lower-parts/buffer-tubes-and-parts/ar15-buffer-system

[Flatland Shooter]

I've got several carbine buffers I can cannibalize for steel and titanium weights, but I'm having trouble locating a source for aluminum weights.

[Patrick Scott]

I think if you poke around the KAK site you can find them  

Edited November 21, 2016 by Patrick Scott

[Les Snyder]

FlatlandShooter... if you cannibalize a rifle buffer you will find hollow aluminum spacers that are the length of two steel weights... I just use a piece of 1/2" aluminum round stock the same length as the steel/tungsten weight I replace...I use an A1 stock with rifle tube and buffers as it offered a lot more options for trials of buffer weights... I've posted this before, but will again... steel weights are 1/2oz

if you remove two steel weights from a rifle buffer and replace with aluminum spacer... 4.2oz (H2) rifle length

fill rifle buffer with #7 1/2 lead shot...6.0oz

rifle buffer with 1/2" brass slug... 6.2 oz

rifle buffer with 5/8" brass slug...7.0oz

rifle buffer poured solid lead... 9.0oz

Edited November 21, 2016 by Les Snyder

[TXMXRACER]

Shot mine for the first time in a match this weekend--using a carbine stock with .223 carbine buffer and spring. Worked great--no problems. Might have to start playing buffers and ammo now. I still have a hydraulic buffer I want to try.  

[Aircooled6racer]

Hello: That KAK kit looks good. I can turn up some aluminum and steel spacers but the tungsten is hard to fit cheaply. Now what spring are you guys using? I will be shooting 130-135PF loads. Thanks, Eric

[Les Snyder]

Eric... I'm settling on a 3.6 WSF 135 Bayou load, squeaks by in the G34 (Briley barrel), about 143 in the PCC.... 5.2oz rifle buffer and stock rifle spring (DPMS)... regards Les

[Patrick Scott]

My load right now is 2.8 N320 behind a 147 bbi. Goes about 880fps out of my 16" gun.

[Aircooled6racer]

Hello: My load for my Mech Tech is 4.1 grains Tite Group with 115 grain Acme at 1.150" OAL. 4.5 grains chronoed @ 148PF. I need to chrono the 4.1 gr and the 3.8gr loads and to see if it will cycle the heavy bolt. I am going to try some 121, 124 bullets as well. I need to order some 147's to try. Thanks, Eric

[Les Snyder]

Eric... my 3.9 WSF 135 Bayou would not cycle my Mech Tech

[Aircooled6racer]

Hello: Shot some 3.8 grains of Tite Group under 115's today. Cycled the Mech Tech just great. I will chrono these on Friday. Thanks, Eric