auto bolt bounce on shorties

Dealers/ Manufacturers/ Regional/ Info Trash
1

Auto bolt bounce as defined by weapon stopping on auto with hammer forward on a live, chambered rd.

Tested a couple combos

  1. two Bushmaster 11.5" heavy barrel flattop shorties/bm lowers
  2. two HK416 10.5" with HK buffer & spring/hk lowers
  3. two LMT 10.5" cqbr uppers/colt lowers
    all have bilock muzzle devices

ammo m193 & m855 milspec, and 77gr .223 NON-NATO spec Black Hills red box not mk262

BM: 1 one BM suffered bolt bounce with regular, H, and H2 buffers with mil ammo types, cannot feed BH 77gr, severly over gassed for mil ammo. Other BM works with H buffer, H2 had cyclic about right, cannot feed BH 77gr

LMT: One LMT has infrequent bolt bounce 1rd in 7 mags with CAR buffer milammo, no issues with H. Other runs fine with all but I put H2 in afterword anyhow for can use. BH 77gr much more sluggish but works.

416 runs fine, superlative weapon on auto, really really sweet 2rd burst a quarter apart at 7-8 yds. Odd recoil impulse on semi, heavier. Almost feels like an open bolt going home. Perhaps because bolt group and recoil spring ATB heavier.

2

Check buffer spring length on the Bushmaster, it should be between 10 1/16" to 11 1/4", if not replace it and try it again with the H2, if it still doesn’t work try a 9mm or H3 buffer.

Keep running an H or H2 in the LMT.

The piston guns usually feel like a heavier more linear recoil (straight to the shoulder with less muzzle lift). I attribute this to the slower cyclic rate combined with a heavier than a D.I. M16 carrier. I don’t know what a HK carrier weighs but a POF and ARES both weigh over 10oz. D.I. guns have lighter carriers and they cycle faster (which makes for less felt recoil, simple physics).

3

all components new (springs etc.) not looking for recommendations, just an informational post for those that carry these 3 common varients. The incident rate of problems with shortie BMs, I’ve seen over last 4yrs, is remarkable.

BTW the HK416 (10") has a very high cyclic rate, higher than the DI guns, the brass is aprox 4-5" apart going out the ejection port. The recoil impulse upsetting the sight picture is the rebound ala bolt going back into battery. As opposed to most weps… esp evident in AK varients where the bolt carrier hitting the rear of the reciever causes sight picture disturbance.

The “simple physics” of recoil impulse, aren’t. If you were to simply define recoil as kinetic energy, in an object that is moving the kinetic energy equals one half times the mass of the object times the square of the speed of the object. In symbols: EK = (1/2)mv2 At some point there will be a shift in EK of heavier weight & lower velocity vs lower weight and higher velocity. Unfortunately this equation does not take spring rate, mechanical friction at varying speeds, friction applied by the user on the weapon, or rebound force. All of which affect the perception of recoil.

4

Spring rates aside…Less moving mass = less felt recoil.

Even better if it’s light mass moving fast vs. heavy mass moving slow.

Think of this way 12ga. 2-3/4" No. 8 shot vs. a 12ga. 2-3/4" slug fired from the same Benelli M1 shotgun, which one will have more felt recoil ? In my M1 the No. 8 recoils less. Part of the equal and opposite reaction since the No. 8 is lighter and moving faster (less momentum) it recoils a lot less than the slugs do (which happen to be typically heavier and moving slower). Your shotgun may do something else.

This is why you see guys using lightened slides, bolt carriers etc in competition (3 gun and USPSA). The recoil will then be less and the muzzle will flip less and allow you shoot on target faster.

From here it reads that a 10" HK416 runs 7-900rpm. A Colt M4 (14.5") runs 7-950rpm which gets even faster with shorter than 14.5" barrels. So one can assume a D.I 10" gun runs faster than a HK416. The D.I guns also happen to have their gas ports erode open which then makes them run even faster. I haven’t heard of this problem with the HK416s.

I have two gas piston guns: 1 Bushmaster 16" (POF system), and one ARES on a 11.5" which I fired on a full auto a few weeks ago with a AK Concepts DNTC on it. On auto the ARES equipped gun could keep a group of about 1.5 to 2 feet at 150yds all the way threw the mag. We shot the same lower with the owners 10.5" and the 10.5" cycled WAY faster than the 11.5" granted thats a 1" difference in barrel it shouldn’t be that noticable. YMMV.

Here’s a pic of the owner shooting his M16 w/H buffer with my ARES upper on it. Next time I’ll bring my shot timer to show the splits between shots between the piston ARES and the LMT D.I upper on the same auto lower.

5

Listen, thats great and all that your a moderator here, but if you want to compare my practical knowledge base, with your theoretical, you’ll find your baseline lacking. In addition to being an end user of all small arm varients in SOF use… I have a bit of a techy/geeky interest easily fulfilled through engineer/manufacturer interface.

Your: “Think of this way 12ga. 2-3/4” No. 8 shot vs. a 12ga. 2-3/4" slug fired from the same Benelli M1 shotgun, which one will have more felt recoil ? In my M1 the No. 8 recoils less. Part of the equal and opposite reaction since the No. 8 is lighter and moving faster (less momentum) it recoils a lot less than the slugs do (which happen to be typically heavier and moving slower). Your shotgun may do something else" Listen fella I gave you the formula, do the math. Your example is piss poor. What is the weight and velocity of the No.8 shot vs weight & velocity of the slug… shot a little lighter eh. And, yes I shot(and have) one of the first five m1super 90’s, inertia action, imported by HK.

I didn’t feel the need to post “certified armorer from sigp226/8 through 60mm mortar” and all bore sizes in Spec War armories in between. The hours spent with Crane and the engineers makes me want to cry. If you for some reason believe the Civ certs are worth anything, I have pieces of paper saying I’m able to diagnose & repair the belt fed & magazine fed offerings from; HK, FNH, Colt, Sig, Glock, etc.

If you really want to get into what makes a recoil impulse the proper formula is one for momentum. This would specifically apply to bolt action or other weapons that dont’t use recoil to cycle the action. I previously noted your presumption that the recoil on the AR is acounted for by rearword carrier assembly movement… whose kinetic energy is easily accounted for by an afore mentioned equation.

The lighter weight used in race guns is only possible by: higher spring rates, reduced power ammo, adjustable gas blocks, longer gas tubes, etc. Throw a lightweight carrier & buffer in a m4a1 and tell me how that works for you. The math is undeniable a light faster object (to a point) creates more kinetic energy.

Your: “From here it reads that a 10” HK416 runs 7-900rpm. A Colt M4 (14.5") runs 7-950rpm which gets even faster with shorter than 14.5" barrels. So one can assume a D.I 10" gun runs faster than a HK416. The D.I guns also happen to have their gas ports erode open which then makes them run even faster. I haven’t heard of this problem with the HK416s."

I don’t care where you READ that an HK cyclic rate is 7-900, I have several next to me that we actually use in real life. Acouple days ago we used them next to CQBR (LMT 10.5") colt lowers. I don’t have to assume anything, I can tell you the 416 cycles significantly faster.

The cyclic rate between 10" & 11.5" isn’t going to be affected by barrel length as you appear to say, but by; ammunition, gas port size, spring rate, and buffer weight.

And BTW the gas ports really only show severe enough erosion to notice when; 1) lots of frange is shot through them(which also destroys 3" forward of the throat) 2) severe firing schedules, when wep stays hot all day as in CQB training blocks

6

…

7

As previously mentioned momentum would be the proper vehicle to calc recoil. m4guru’s stated notion that lightening the carrier group somehow translates to less recoil is how Kinetic Energy (EK) came up. A more appropriate way to measure mass striking the rear of the reciever extension aka buffer tube.

8

…

9

I’ve only shot one factory 416 and one 416 upper on a Colt M16A2 transferrable lower. In my experience, which obviously can’t compare to yours, the factory 416 does cycle more slowly than a 10.3" Noveske using an H2 or 14.5" Colt M4A1 cut to 10" a la the Mk18 using an H2.

Matching your experience, the recoil in both semi-auto and full auto does not mimic the recoil in DI guns. I, and you, don’t need to offer hypotheses, as it’s pretty widely accepted as fact.

Bolt bounce is a pretty common occurence in most M16 derivative machine guns I’ve shot. Very few owners who are shooting “aftermarket” uppers that are shorter than factory offerings, haven’t had to do some “tweaking” to cure bolt bounce, super fast cyclic rates, etc, etc, etc…

Especially when using the hotter ammos like 3131A, M195, M855, Mk262 or 75gr TAPN.

So I really must ask now, how many of the toys you use are yours to take home and keep forever?

10

Dport

Of course were talking about “subjective” perceived recoil, what other kind is there?

Equal weapons “mass” weight = recoil… not really. I have a g3 18" that weights the same as a m14 18"… and the g3 recoil bites. These two different operating systems, have dramatically different recoil characteristics.

The “mass” of the weapon, while making for an easy equation, does not take into account:
-the energy used by assorted operating systems
-the recipicating mass slamming about the reciever
-Chamber pressures, duration(barrel length) of gas burn, (controlled explosion)
-what is the expanding gas doing while leaving the bore… devices
-surfaces transmitting recoil to shooter

11

You can disagree with someone, but there is no need for all that.

12

…

13

I agree. I have zero experience with the HK416, but I do have a lot of trigger time with the LMT 10.5" (mine) and the BM 11.5" (BIL’s), both are our work guns.

The weapons are semi-auto and both have a rail, foregrip, light, optic and buis. The BM has a s/a BCG w/ carbine buffer while my LMT has a f/a BCG w/ H buffer. The Bushmasters cyclic rate is faster do to it being over-gassed, as are most BM 11.5’s I’ve shot and been around. The recoil is also significantly sharper than the LMT’s despite having the longer and heavier barrel.

I gave my BIL a H buffer, along with a Crane-kit (because I think every shorty should have it) to try and tame the beast. The heavier buffer helped a little bit, but it is still nowhere near as “smooth” as my LMT.

14

“pure momentum” funny. I think I’m done here. I see the same little rolly eye BS I would expect from the usual type suspects hiding behind their keyboard. In the unlikely event you meet actual end users, please feel free to step up and roll your eyes while in the house, or on the mat.

Have the best day ever!

15

Well, if you want to talk one on one, PM me and I’ll give you my phone number. I did the “rolleyes” face because we were making the same point. You seemed to care more about the argument than understanding my post.

Sorry you were so easily offended by that.

16

I think you need to chill out there mister know-it-all :rolleyes:

17

Nice…

This kind of crap reminds me of TOS…

18

Okay guys – keep it professional and impersonal.

19

:smiley:

You will always get stupid replies!

And some guys will disagree no matter !

I see this alot on the OTHER Forum

20

…