Gunz - Wow – very impressive !


Ok, a couple of questions;
I don’t have an engineering background, so these questions may be flawed.  Please bear with me.  

How do you interpret these results to the theory of the flex causing alignment issues with extension and bolt resulting in broken bolts ?

Maybe I am wrong (wouldn’t be the first time ), but isn’t 20lbs a huge amount of weight to replicate downward pressure in a real world scenario ?   Or is the change in the indicators at 10lbs or less lbs enough to make the same conclusions?

Wouldn’t the farther forward you put the weight on the handguard the more flexing you would see ?  On the tests it is at 7.5”.  Carbine handguards are about 7” in length and a vertical grip would generally be located at about 2”-3” ?


thanks in advance !

So the "piggy back" adds some of the rigidity that was lost on the flat top when compared to the A2 type fixed carry handle upper?

A few random thoughts/points.

I agree that the receiver flexes....But does it matter?

The reason this has been a hard issue for me to really blame bolts braking on stems from my service rifle and long range shooting experiences.

For years, match AR's have been built using 24-26" heavy match barrels.  These rifles also sport FF tubes of various manufactures.  All this weight gets 'hung' on the end of an alum receiver, that flexes.  Now add that anyone shooting an AR in a service rifle match or across the course is using a sling with enough tension to make an an unprotected hand go numb (we wear shooting gloves).  I would even think its fair to say that most non service rifle shooters would be really shocked how tight some shooters get their slings.

I doubt there is a good scientific way to measure the deflection but I will tell you its visually apparent on my service rifle when I get locked in to shoot prone.  My service rifle is old colt Hbar with a merchant stainless FF tube under the HG's.  It shoots under .8 MOA (it refuses to die) and has had thousands of rounds thru it since I got it over 10 years ago (used).  Has it seen FA abuse cycles? - NO.  Has it seen a few mag dumps (especially in my younger days)? - you bet.

I'll bet I put at least the same deflection as a 20 pound weight at 7 inches.  I would also think the flex would be worse on a space gun with a much heavier barrel under the same conditions.  Especially when one considers the 'preload' of the heavier barrel.

So the questions is why dont service rifle shoots and shooters of match rifles brake more bolts?

Certainly, they dont subject their rifles to FA abuse cycles - unless theyre rich or retarded or both, but they do shoot alot - more than most recretational shooters.

So what we really need to figure out is how much flex is enough to cause problems.  Some  test method where we look at bolt binding issues under differents amounts of flexion - to be a really good test it would have to be done under real conditions, ie. actually firing the rifle.  Realistically best we can probably do is to look at the results when we just cycle the action dry.

Moreover is there some point in assembly that can improve or fix it, if its a real issue? And lastly, is this a materials issue or a design issue?

Guns brake, parts wear, its life.  Ive broken 1 AR bolt lug at 500 rounds (Colt factory gun, totally stock other than the trigger), a glock extractor at 56 K, a sig trigger pin at around 30 K, but is it a design fault?  A materials defect? or just a result of wear and tear?

Great job, by the way.  You really did a nice job.

My only concern is whether this is 'real' issue or just another 'the sky is falling'/'buy my new better upper' thing.

Quoted: I'm glad to read this.  I was flamed a while ago on another thread when I talked about USASFC (US Army Special Forces Command) changing out all the regular M4 profile barrels to the new Heavy barrels because bolt lugs were being sheared from pressure exerted using the forward grip and barrels were bursting at sustained rates of full auto fire.   Would the fact that the knights armament SOPMOD kit is not a free float rail have any additional stess added to the upper or barrel nut area causeing the lug shearing?

That has been one of the competing theories in some circles, but is that the conclusion we should come to after reviewing this data ?

Is a 20lb weight (pulling straight down) at 7"+ out on the barrel representative of what the carbine would experience while being shot by a soldier ?
Does the 10lb data more closely correlate with "real world effects" ?
Would the change in the indicator measurements be enough to cause misalignment in the bolt and barrel extensions that would cause damage ?

Gunz performed the testing and collected a lot of great data.  The next step is this data needs to be interpreted so it can be applied in a "real world" sense.  I will let men smarter than me do that.