Buffer selection in 10.5" LMT carbine

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Hey,

So I'm building yet another Mk18 clone but had a few questions about which buffer to use. My LMT lower came with a carbine weight buffer and I understand that LMT even ships its completed short carbines with a standard weight buffer.

However, is it better to run a H or H2 with the 10.5 upper?

Also, is it true that LMT 10.5s have a smaller gas port than Colt so they are more optimised for the short barrel and hence can get away with a car weight buffer?

If you have the gas port dimensions for the LMT vs Colt, I'd like to see 'em.

Thanks!
 
You can probably get away with the H, but the H2 should be ideal. I've run 10.5" with no problems using a full auto bolt carrier and an H buffer. If a semi-auto carrier is to be used, I'd probably opt for the H2 just to be safe. That said, you might be lucky enough to have a rifle that will run just fine with the standard carbine buffer. LMT build good rifles. I haven't seen or heard anything concerning a difference in gas port dimensions between LMT & Colt. As a government contractor, I would surmise that LMT would make it a point to ensure that as many specifications as possible were standardized in order to preclude any potential mix-up when it comes time to make delivery on said contracts. It is a WAG on my part, but I'd say the dimensions are the same. I know that they are on my 14.5" M4 profile LMT.
 
Yes, I have a standard LMT bolt carrier, not F/A (is there really a big weight difference?- only so much kool aid that I can drink).

How do I tell which buffer to use? Should I keep increasing buffer weight until the gun short strokes, then back the weight down by one?
 
Try the buffer that shipped with the rifle. If you don't have any cycling problems, all well and good...no need to go looking to fix a problem that doesn't exist. If you do experience cycling problems, FTF/FTE, pick up an H2 and you should be covered, even with the standard semi-auto bolt carrier.
 
Wouldn't FTE/FTF be from shortstroking? If so then slowing down the cycle with a heavier buffer would be detrimental, no? I thought the point of the heavier cycling parts was to slow the cyclic rate and save the gun excess abuse. Please correct me as I may have gone in the wrong direction with my 10.3" build.
 
You are making your life way too complicated!

If the carbine work with the regular buffer - why fix it? I won't worry about the SA/FA carrier at all.

I have been using H for years in LMT 10.5, before everyone needs to have a 2H. H buffer is the standard for the MK18, so is the M4
 
skooch said:
Wouldn't FTE/FTF be from shortstroking? If so then slowing down the cycle with a heavier buffer would be detrimental, no? I thought the point of the heavier cycling parts was to slow the cyclic rate and save the gun excess abuse. Please correct me as I may have gone in the wrong direction with my 10.3" build.
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the heavier buffer is to increase the energy in order for the extracting cycle to initiate. The gas port of the MK18 is smaller than most commercial offerings because it is optimizef for the military ammo. A big gas port increases the volumetric flow and hasten up the initiation of the extraction cycle. The earlier this starts, the harder the extraction - commercial systems use bigger gas ports because there is a wide range of down powered .223 in the market that the consumers choose from, whereas the military has a much narrower range of ammunition spec.

Some people build on the idea and start adding 2H, 3H....and backdown until the rifle stops working. IMHO, this is completely invalid. You should use the lightest buffer you can get away with the ammo having the highest port pressure in the hottest weather. This will ensure you the most margin with the same ammo in cold weather and other ammo with lower pressure.
 
So if you get FTE/FTF then increaseing buffer/bolt mass is useless. If this is happening increasing to a H of H2 will not solve the problem. Was X-man backwards? I'm just tring to get my head straight as I thought I understood the mechanics behind it all. If not I must give my head a shake. :p
 
I did a little experiment with my 11.5" DPMS barrel. I started off with a standard carbine buffer (85g) and a standard bolt carrier. Then I began adding weight. I got all the way up to 305g of weight and the rifle kept cycling just fine with 55gr Am Eagle ammo.

With all the extra weight the recoil impulse definately feels softer but the rifle loaded, ejected and locked back just fine.

Because it is a cheap and easy thing to do, I intend to run as much bolt carrier/buffer weight as the rifle will accept. IMO this will reduce the battering of parts and make my expensive rifle last longer.
 
when the system the buffer and bolt group is moving rearwards, the energy is transferred into energy within the compressed spring - excessive energy is transmitted as kinetic energy of the entire system and others. The spring is not going to store more energy because of the buffer weight.

When the spring is fully compressed - it is storing certain amount of energy depending on the spring rate.

The energy released when the buffer and the bolt going back into the battery is the same irregardless the weight of the buffer.......the principle of the conservation of energy. A light and fast hammer could transmit the same energy as a slow and heavy hammer.

Therefore, the idea of using a heavy buffer weight to reduce "battering" is not valid.
 
I will disagree... a heavier buffer will slow down the RATE of rearward speed of the buffer/BCG, all else being equal, since F=MA. Same force from the gas tube, but a greater mass, so acceleration of the assembly rearwards is reduced.

I agree about the spring conservation of energy, but that would be a steady state issue (non time varying, i.e. a constant force from the gas). The force comes in a limited time pulse, so the load is transient. So you have to factor in the dynamic loads, and if the acceleration rearwards is too slow due to a heavier mass, then the gas load will disappear before total rearward travel is obtained, causing a short stroke to the rear.

So shortstroking (I mean the BCG and all not completely running to the furthest normal rearward limit) could happen if your buffer is too heavy (i have no idea what the actual threshold would be and would depend on how dirty/gummy the action is).
 
Therefore, the idea of using a heavy buffer weight to reduce "battering" is not valid.
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Would you rather be hit by a very light bullet traveling at very high speed or a very heavy bowling ball traveling at very slow speed? Personally I'd take the bowling ball everytime.

Bolt carrier velocity will make a difference to the beating or not the rifle receives.
 
The AR-15 gas system was never intended to work with short barrels. Adding a heavier carrier and/or buffer does have a positive impact on ensuring reliability for "shorty" ARs. It also tames the "violence" of the much shortened gas system cycle and should extend the life expectency of the rifle. You don't have to go crazy with the extra heavy buffers, but as I recommended above, go with the weight that gives you the best performance in terms of functionality. I've built a number of shorties on different lowers/uppers and this is what I have found through trial and error.
 
LMT does use relatively small gas ports on their 10.5s ( about .071" ) . I have measured my Noveske 10.5 and WOA 10.5 to .079 / .080 for comparison

No problem with H buffer or H2 as long as you run NATO pressure ammo. It can short stroke with SAAMI pressure. When I spoke to LMT they informed me that they use a relatively small port because it runs well with a supressor AND NATO pressure ammo.

I don't run SAAMI ammo so its not a problem. Only issues I have had is with a Enidine buffer ( weight between a H2 and H ) it sometimes does not lock back on empty mag if I neglect the lube / cleaning. My NON LMT shorties don't have this problem

It doesn't help I'm also running a relatively heavy carrier
 
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