Best Buffer for a 5.56/9mm lower

[ecampo]

Recently finished a 7" dedicated .22 build and want to add a ~5" 9mm and 10.5" 5.56 upper as options. I've never run a build in either length so I don't have any experience with what type of buffer to use. Basically just looking for if there is buffer will give me the best of both worlds or if i will need to swap with each caliber. Doing some reading it sounds like 9mm buffer will allow the use of both but I'm looking for some input from people with builds around these lengths. I realize a dedicated 9mm lower setup would be the best option and that will probably come down the road, but don't mind swapping around some parts for now.

 

[dontpanic]

Get a Rock River 9mm buffer. It should run both quite well. It's only $25 so if you have to go with a heavier buffer like the Spikes or Q buffer, it will be no

 

[DYI01]

You will need specific buffers for both calibers. A good dedicated 9mm buffer will be about an inch longer than a milspec 5.56 buffer to account for the case length difference so you don't break bolt catches. Running a 5.56 buffer without a dedicated 9mm buffer or spacer will let the buffer get a running start to slam into the bolt catch on the last round of the magazine causing the breakages. I would also recommend an extra power spring for the 9mm to go with the heavier/longer buffer. Since the 9mm is a blowback design, it will help keep the bolt closed for longer and keep bullet velocities up.

 

[erwos]

You _can_ run a 9mm buffer using no spacer with a 5.56 upper, especially if the upper seems to have robust gas flow with your chosen ammo.

The problem is that for optimal functionality, you really want a separate buffer for each caliber. 9mm tends to run best with super-heavy enlongated buffers (think Slash's 9Q), whereas going beyond H3 on a 5.56x45 gun is probably just adding weight and causing reliability issues.

Between the buffer and the magwell situation, I tend to think going dedicated with 9mm ARs is a better bet overall, unless there is some compelling reason not to (ie, you have a M16 RR MG).

And just as an FYI, a 5" 9mm AR-15 will not meet OAL requirements to avoid being a copycat, so if you're using a post-10/13 SBR lower, you need to be careful what you're putting on there.

 

[clandestine]

You will need specific buffers for both calibers. A good dedicated 9mm buffer will be about an inch longer than a milspec 5.56 buffer to account for the case length difference so you don't break bolt catches. Running a 5.56 buffer without a dedicated 9mm buffer or spacer will let the buffer get a running start to slam into the bolt catch on the last round of the magazine causing the breakages. I would also recommend an extra power spring for the 9mm to go with the heavier/longer buffer. Since the 9mm is a blowback design, it will help keep the bolt closed for longer and keep bullet velocities up.

This man is correct. Buffer length is very important on a 9mm AR to prevent bolt catch breakage. The best spring to use for the 5.56 and 9mm is the Chrome Silicone Flat Wire Spring made by Superior Shooting Solutions. Just use it, don't cut it.

As far as what buffer to use for a gas operated ar, it not possible to say. It depends on way too many variables such as gas port size, gas system length, type of carrier, supperssed, unsuppressed, and ammo choice. If you pick one type of ammo and shoot it without a suppressor all of the time you can pick one buffer and spring and be done, but with countless choices in ammo and too many weapon variations it won't be possible.

Best option is to get a click adjustable gas system and then document what ammo combinations run on what setting.

 

[j_h_smith]

Reliability is what you're after and I'm not sure you will get this with just one buffer. Build a dedicated 9mm lower (I recommend CMMG) with the heavy buffer and you will never look back. There is also no worries with the copycat law if it's a dedicated 9mm lower.

Good Luck!
Jim Smith

 

[ecampo]

Good info, thanks everyone. I will probably get it tuned for a 5.56 upper and wait on the 9mm until I build a dedicated.

 

[IMBLITZVT]

...The best spring to use for the 5.56 and 9mm is the Chrome Silicone Flat Wire Spring made by Superior Shooting Solutions. Just use it, don't cut it....

Is that spring going to increase cycle rate in FA applications? Seems like it has slightly increased spring pressure with bolt closed.

 

[clandestine]

Is that spring going to increase cycle rate in FA applications? Seems like it has slightly increased spring pressure with bolt closed.

It may slow it down but the good thing for a M.G. is reduced carrier bounce which can cause timing issues.

Check out David Tubbs video about that spring if you haven't seen it. Normal Springs last 5k cycles before they go below the spec, but these springs will last 500k cycles. I also use their flat wire Magazine Springs as well.




BTW, congrats on your R.R. Its a beauty. I'm looking to pick one up too to use for my AR Classes.

 

[IMBLITZVT]

It may slow it down but the good thing for a M.G. is reduced carrier bounce which can cause timing issues.

BTW, congrats on your R.R. Its a beauty. I'm looking to pick one up too to use for my AR Classes.

So with increase spring load when the bolt is closed, the bolt carrier will be slower to accurate back. However it will also close quicker too, correct? So what does this do to the time? Why are you sure its going to slow it down. I watched that video. He says follow up shots are quicker. However I was assuming that was because of quicker cycle time, not slower cycle time... if related to that at all. I guess he could be talking about the reduced felt recoil? Yes less bolt bounce is good but does not seem like a huge issue in this gun like you sometimes hear about in roller delayed blow back guns.

Thanks, it was painful to do but now that its done, I am glad to have it. It just opens us your shooting solutions so much!