MFT mag holds bolt open on empty ONLY if I shoot slowly. Why?

LAV

LAV

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I have an M&P Sport II. I have been tuning an adjustable gas block so I have been shooting from MFT LAR 15 mags that only have 1 round. The MFT mags have extensions for a better grip. Shooting with the mags with 1 round in them locked the bolt back (when it had the right amount of gas coming from the adjustable gas bock).

However, if I put 2 rounds in the mag and fired them in quick succession (say, two shots in 0.5 seconds), the bolt would not stay open when the mag was empty. This happened even with the gas block opened full.

Then, I loaded 2 rounds in a CPD LAR 15 mag and the bolt stayed open on empty every time.

Someone please tell me why the MFT LAR 15 mags are not holding the bolt open on empty during rapid fire. Thank you.
 
I'm guessing probably a weak mag spring or mag follower is sticking or buffer weight too light or buffer spring is on the strong side or combination of all or some of them?
 
The fore-end was rested during shooting. I hold my rifle’s free float rail when I shoot it (not rested).
 
LAV said:
Someone please tell me why the MFT LAR 15 mags are not holding the bolt open on empty during rapid fire. Thank you.
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After a couple of days I have been able to think up only one mechanism that could explain this.

The common forms of AR-15 buffers have a polyurethane bumper at the back. If the buffer travels full stroke to the rear of the buffer tube this bumper cushions the blow to the back of the buffer tube.

It is characteristic of polyurethane that it gets stiffer with repeated impacts in a short period of time. If there is a second or two between impacts the material has time to relax, and will behave consistently. If the material is impacted multiple times per second however, the modulus of elasticity becomes progressively higher with each hit. This reduces the cushioning effect of each subsequent blow, and the bolt bounces with higher energy, so that it's return velocity on each return stroke is higher than the previous one. This results in increasing cyclic rates of M16 rifles, I think the phenomenom is described in The Black Rifle somewhere.

It it possible that your hold open is barely engaging fast enough under good conditions, and the decrease in cycle time as the bumper hardens is enough to bugger things up. If this is true your possible fixes are: debug the mag follower and bolt hold open so that they move faster; and/or use that adjustable gas block to slow the bolt down so that it isn't hitting the back of the buffer tube so hard.
 
Kidd X said:
The only variable is whether or not the mag is built to spec or not.
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Not quite how variables work, but like many other threads, you will endow us with your endless wisdom...

For example, if the magwell is out of spec, a mag that is rocked forward would not hit the bolt catch... but those of us that have been around a few years might never have know about that. You know... since the op said the mag extensions are installed.

Sure, it is most likely mag related, but that doesn't discount anything else until you look into it.
 
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What was the temperature? My MFT mags work ok above zero, but will double feed almost every time at -10C or below. USGI aluminium mags work fine regardless.
 
Daver_II said:
Not quite how variables work, but like many other threads, you will endow us with your endless wisdom...

For example, if the magwell is out of spec, a mag that is rocked forward would not hit the bolt catch... but those of us that have been around a few years might never have know about that. You know... since the op said the mag extensions are installed.

Sure, it is most likely mag related, but that doesn't discount anything else until you look into it.
Click to expand...

You're right, the op has a sport II and mft mags are far from being premium mags. Based on those facts and the post by 5.45 it could be both rifle and mags that are the problem. I would start with the cheapest solution and get different mags.
 
Battlerifle, thanks for chiming in. I recently swapped the factory buffer for a JP SCS Gen 2. I am running their standard spring so not the light or heavy one. However, I was running some drills before with the factory buffer and I was having the same issue.
 
Have you tried giving it a little more gas?
When tuning my RFB gas system I would single load and reduce gas till it wouldn't lock open any more then give it two or three clicks more gas to not only get it back up to where it would cycle reliably but also gave it a little extra to compensate for when the rifle started getting a little dirty.
Same principal as when I handload subsonic for my 300 Blackout, I reduce charge till it stops locking back then go back up a couple steps past where it was locking back to ensure that it cycles when it gets dirty and also in case I rapid fire and basically for lack of a better term limp wrist it but unlike a pistol, in the case of a rifle when you rapid fire and aren't using good form the butt of the rifle is bouncing on your shoulder rather than being tucked in solid.
Typically an AR is overgassed and it doesn't matter if you use poor form but if you reduce the gas to close to the minimum it needs you need to be more consistent in your form which is harder during rapid fire.

If more gas doesn't fix it I would guess the MFT mag has a little weaker spring and isn't pushing the follower up fast enough to catch the bolt.
 
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cr5, I will try that. It boggles my mind that three different metal CPD LAR15 mags run flawlessly and the MFT mags do not hold open on empty on rapid fire.

I am also going to try the MFT LAR15 mags without the extensions as the extensions snap where the original baseplate sits. I measured the depth of the original base plate and the extensions’ and they seem to be the same depth so it looks like the springs are compressed the same. I will test it anyway to eliminate a variable.
 
Are the MFT mags 5/30 or a 10 round pistol mag? Sorry, I'm not familiar with the MFT mags, I run Pmags and USGI metal mags pretty much exclusively.
Since is cycles fine with Cproducts mags I would try removing the spring from the MFT mag and stretching it slightly. If it resolves the problem you need longer/stiffer springs.
Another thing if they are 5/30 is to check that the rivet is not interfering with the spring. Some importers do a crappy job pinning the mags. If the rivet is too long the spring can catch on it stopping it from applying enough force to the bottom of the follower. If that's happening just drill the rivet and replace with a slightly shorter one. If they're polymer mags go slow drilling or it will melt the plastic and the new rivet won't hold. Also be careful you don't go too short or the follower will be able to get past and you'll risk having an illegal magazine.

If you're in the Edmonton area and need some help PM me and we can try to arrange a range day and I'll give you a hand. I've fixed plenty of mags over the years, there usually isn't anything wrong with the magazine but our stupid mag limit laws create reliability problems by inserting foreign objects into the mag that interfere with function. It can be done but some importers do a pretty poor job of it.
 
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