Okay,

I have a question concerning a buffer.

I am building two retro 5.56 carbines with pencil weight barrels.  
One will have a 14.5 inch barrel (with a perm attached FH) and the other will probably end up with a 12.75 barrel (Once I have a 16 inch cut down) with one of Bricks XM177E2 moderators perm attached.

Question is.... What weight of buffer should I run in each of them?

I have a H2 buffer and an H3 buffer already.  I have been told that the H3 buffer will work well and slow down my action causing less wear and tear on my BCG/FCG.

I just wanted to see what you way more experienced guys used in your existing retro carbines.

Thanks
Spence

Run the weight that works?

That's really the informed answer.

If a standard carbine buffer is working, you don't need a heavier one.  Remember the heavier buffers were really intended to slow down the cycle for burst and automatic fire; in semi there's more than enough of a pause while you reset the trigger for the bolt to stop bouncing.

In my 11.5" I'm running a 9mm buffer because it seems to like it better than standard.  The 14.5" is happy with a standard weight.

To start what diameters each barrel's gas port will be?

The M4 (14.5" barrel) carbine gas system runs a 0.062"/0.063" gas port diameter, and NSWC-Crane set the gas port diameter of the Mk18 (10.3" barrel) with a carbine gas system at 0.070". The XM177E2 (11.5" barrel) had a 0.067" gas port diameter, and the XM177E1 (10" barrel) had a 0.072" gas port diameter (XM177E1 & XM177E2 both had carbine gas systems). The M16A1/2/3/4 (20" barrel) uses a gas port diameter measuring 0.092"/0.093". As for a mid-length gas system with a (16" barrel), the ideal gas port diameter is around 0.078". If we place these barrel's in a list, you'll notice a pattern develop.

10" (carbine) 0.072"
10.3" (carbine) 0.070"
11.5" (carbine) 0.067"
14.5" (carbine) 0.062"/0.063"
16" (mid-length) 0.078"
20" (rifle) 0.092"/0.093"

Now, if you are wanting to precisely tune the buffer and action spring setup page 84 of the PDF (page 74 of the document); Final Report on Product Improvement Test of Submachine Gun, 5.56-MM, XM177E2 dated June 1968 has math formulas that can be to used to end up figuring the weapons rounds per minute. Because if the weapon falls below the minimum rounds per minute, this is when short-cycling happens. Which occurs when the urethane bumper on the buffer only touches the rear of the receiver extension without compressing.

This is as topic of particular interest to me since I machined my own buffer, using Urethane/Delrin/tungsten carbide weights, for my 607 project. I think the correct answer is to use the HEAVIEST buffer you can, and still have the action cycle without short cycling, and lock open after the mag is empty. IF you use a std weight carbine buffer there is no doubt that your rifle is going to run. The only problem is that it is going to run hard, recoil sharply, and needlessly wear components of your AR. Not everyone has an assortment of buffer weights, nor wants to buy an assortment, but since you do you will want to run the heaviest one that still lets the action cycle reliably. The heaviest that you can reliably run will smooth out the recoil impulse and allow for more accurate/faster follow-up shots. If you use the H3 and the action short cycles, then put in the H2 and see what happens.  The only caveat is that of ammunition. If you are using 5.56 Nato ammo, and it runs fine with an H3 (just as an example) and then use .223 ammo, there is the possibility that it might short cycle. As the power of your ammo goes up, then the weight of your buffer can be heavier........conversly, the weaker your ammo (pressure wise, and .223 is lower pressure than 5.56) then the lighter your buffer needs to be.