Front to rear play in AR.

[HammsBeer]

Ive searched for a while and all I find is concerns of side to side play in the upper to lower receivers in the AR. Im not worried about that, my concern is a very significant front to rear slip between the upper and lower receivers, and the possible stresses of when it fires and the recoil of the upper receiver impacting the lower. I can visibly see the upper sliding back and forth on the lower, maybe about 1 millimeter or so. No visible damage, no malfunctions and both are the same manufacturer.

 

[kimbernut]

Forward,rearward movement

I'm not understanding how that would be possible with the takedown pins in place. Am I missing something here?

 

[allaroundhunter]

It sounds like you have a small pin upper matched with a large pin lower. You need to get offset pins to remove the play. Or do yourself a bigger favor and get a small pin lower instead.

 

[AlexanderA]

Sounds like the takedown pin holes in the upper receiver are elongated or oversize. If so, the fix would be to drill them out larger and bush them. This is a job for a gunsmith.

 

[HammsBeer]

Ok, thank you for the responses. After getting home and looking at it again, it's more like .5 mm, but I'll contact the manufacturer. Thanks!

 

[hentown]

Something's definitely wrong. I've built a bunch of ARs, using several different brands of lowers and uppers. All of my builds lock up tightly.

 

[HammsBeer]

Ok I'd like to revisit this topic so bare with me...

I finally measured the front to rear slip in my receivers with a mic, and it's about .005 of an inch (sorry my judgement of a half milimeter is a little off). I can definately feel it with my finger at the front pivot pin.

Looking at the AR design the initial recoil of the upper receiver travels through the front pivot pin to the lower. Then the BCG cycles and bottoms out in the buffer tube, and then moves forward to chamber and pushes the upper receiver forward. So the pivot pin is being forced back and forth in the pivot pin holes, correct? That seems like a very small amount of material to take that much pounding. If the upper is moving even that small .005" distance, wouldn't it eventually cause the pivot pin hole to get wallowed out or elongated? Or is that .005" play not detrimental?

The pivot pin holes are .250" but most pivot pins are actually .247" which may account for that .005" movement. I see that Armalite has national match pins that are a true .250, so any thoughts on using those to take up the slack? Sorry for being OCD, but I'd rather fix any possible issue now than wait til it's too late.

 

[pretzelxx]

The colt m4 has some play, can't measure at the moment, but I have yet to see any lower receiver issues. Out of about 3000 inspected m4s, I've seen one cracked upper. This was an IED hit weapon, not normal wear and tear. I would consider it a very small thing to worry about. Mind you, millions of rounds have been fired through all these weapons. Some have burned three barrels in my three years here, I've looked up the paper trail on a few of the weapons.

 

[MrCleanOK]

Sweating over .005" is not worth your time or energy.

 

[HammsBeer]

Well when you can slide the upper front to back and hear a "clink clink' it can't be doing the rifle any good to shoot with that play.

We went to a gun show today and handled almost every brand of AR there. None had the play I'm talking about.

 

[Jackal]

We went to a gun show today and handled almost every brand of AR there. None had the play I'm talking about.

Then clearly yours is messed up, send it back for warranty work. Or, just shoot the crap out of it.

 

[Ridgerunner665]

Replace the pins...may or may not help...but its a small price to pay to find out.

That said....I agree, its a non-issue.

 

[Tirod]

I've been issued used M16's that had so much slack in the upper and lower that you could pass a playing card between them. Yet, they could still shoot Expert.

The action reciprocating in the upper has less recoil affecting the lower than a locked breech gun. The action unlocks and the bolt carrier moves backward using some of the force of the gas expansion as the case pushes against the face of the bolt. That force propels the carrier backward and the buffer collapses the action spring against the back of the buffer tube.

All that spreads out the affect of the recoil impulse which lowers it relative to the abrupte push of recoil in a locked breech gun. And in 5.56 it's substantially lower, too, than a .308.

Point being, after a decade of use and hundreds of thousands of rounds, the M16's in Basic keep on taking the beating and don't fail. They are used in bayonet training, they get pogo'd to free stuck rounds by literally stomping the charging handle, they lift a third soldier in full battle gear while the other teammates hold the butt and barrel.

Those action pins and lugs can take a lot and a little play isn't a big deal. I'd rather have one a tad loose than tight. It doesn't affect accuracy whatsoever, and it's not going to hammer the lugs or elongate the holes. The lower trigger pins are more likely to see that and it's why full auto lowers are refitted with special ones to keep it running.

If there is something to discover about the M16, it's that it is definitely not subject to the same rules as a manual action locked breech gun - because, it isn't. It's a self loading action and it's even more different in that the receivers are not highly stressed parts. The barrel extension does that, which is the real design landmark that makes the M16 different.

Anyway, my pins are so tight I still have to hammer them in and out. You can have too much of a "good" thing, too.

Just shoot it.

 

[HammsBeer]

UPDATE....

So I tried swapping uppers and lowers with someone else's AR. My upper was tight on their lower, and their upper was tight on my lower. What gives!? Even he thought my receivers had excessive play.

So, convinced is just tolerance stacking in the pivot holes between my upper and lower, I ordered a set of Armalite national match pivot and takedown pins. My old pivot pin measured .2470", the NM pins measured .2495". A little bit of grease, pushed the pivot pin through, and presto no more front to rear play! The pivot pin takes some firm thumb pressure to remove, but still doesn't require tools to pop out. The slop is gone so I don't think I'll need to install the takedown pin, they were just $10 each but good golly they wanted $14 to ship two pins! Oh well, at least my slop is gone!

 

[Tirod]

If you feel better about it then the money was worth it to you. Swapping the uppers and lowers around was a good choice and certainly education. Very typical, and exactly why some makers can do a better job than others when they mike the batch of parts to assemble with corresponding ones. That is where "QC" that the public expects shows in a gun.

The irony is that 10,000 rounds from now, that receiver set will more than likely be loose again. And it still won't make any difference in accuracy.

 

[12many]

I don't think it matters. Some guns are not tight. What is the brand or did you build it? Has someone mentioned that little rubber thing to tighten it up? I can't remember the name.

 

[goon]

I think you're thinking of an accu-wedge.

All three AR's I've owned had a little play and so did the M-16's I was issued. But if adding the oversized pins gives the OP more enjoyment from his rifle, then they were a good investment.

 

[12many]

yes accu-wedge. I have seen several gun show rifles in which they were installed to make the gun tight and the seller can brag about how great the fit is even though if you take it out, it has some play.

 

[WinThePennant]

All my ARs have some 'play' in them (if you are getting out the micrometer -- YES, they all will).

I am going to give you the same advice I give anyone who frets about a non-serious "defect" in their gun.

1. Take it out and shoot the piss out of it.
2. Smile from ear-to-ear.

 

[HammsBeer]

Yes I realize they all have side to side play and that is perfectly normal and I don't care about that. My issue was a FRONT to REAR play, inline with the recoil forces. They were from the same manufacturer but bought at different times, so they were not fitted together. Like I said, fitting them to other uppers and lowers showed no issues, so it's a tolerance stacking issue (google it).

Let me explain how this happens:
Look closely at any upper receiver and you will find the front pivot pin hole is an exact .250" but the rear takedown pin hole is purposely machined a bit oblong to allow for slight discrepancies between manufacturers, and most of the time the takedown pin ends up resting against the front or rear of that hole (ever wonder why some uppers and lowers fit tighter than others, thats why). My rear takedown pin goes through the exact center of the rear hole of the upper. What this means is that any front to rear play is strictly controlled by the front pivot pin. Since the front holes are .250", but the pivot pin is .247", this creates .003" of play in the upper's pivot hole, and another .003" of play in the lower's pivot holes, thus a .006" front to rear play. By replacing the .247" pivot pin with the NM .250" pin, that stacking tolerance gap is removed, and creates a tighter fit.

Yes, you could acheive the same effect with an accuwedge or front o-ring, but that is simply forcing the receivers to the opposite ends of their tolerances. Why not fix the tolerance gap in the first place instead of bandaiding it?