I just took my new baby to the range - a CavArms/Ameetec midlength build:



The barrel is Govt profile under the handguards, 5.56 chamber, chrome lined, 4150 steel, 1/9 twist.

First, I zero'd at 200yards using slowfire 5-shot strings off a sandbag (I used Q3131A, but the rifle groups well with XM193 too). After zeroing at 200yds, I let rip with a couple of mags in a couple of minutes. The barrel got pretty hot (obviously) plus it was 103degreesF here today. With the hot barrel, I found that my point of impact shifted upwards by about 12 inches at 200 yards !

Most of my 3-gun shooting is done with a heavy barrel and freefloat tube, so I haven't really seen this magnitude of shift before. Is this amount of zero shift normal for a non-freefloat Govt barrel ? By the way, when I let the barrel cool down, the zero returned to normal.

WOW!

That's quite a bit of movement.

I can't really help you out though........I'm just keeping this topic up top so you can get this answered

WIZZO

I have experienced 2-3 inches at 100 yds, so 12" at 200 isn't that much more.  How hard were you leaning on the gun?

Are you sure the barrel wasn't resting on something? That can change the harmonics of the barrel and cause the zero to shift pretty noticeably. I've never seen a 6 MOA shift from heat though, even with pencil barrels.

The barrel was not directly on anything, but the handguards were resting on the sandbag. I was not leaning on the gun too hard, but I guess some force is inevitable.

Quoted: The barrel was not directly on anything, but the handguards were resting on the sandbag. I was not leaning on the gun too hard, but I guess some force is inevitable.

Any force on the barrel after it is heated up that much is going to effect it, and a substantial shift like that is believeable.

When the handguards are resting on the sand bags, the weight of the gun is pushing the handguards down on the sand bags, which in turn make the handguards push up on the barrel.  Dpeneding on how tightly you are pulling the rifle in and down into those sand bags to steady the rifle will play in to your POI shift when the barrel heats.

A FF handguard can definately improve the consistency and accuracy of a hot thin barrel.


edited to add:  BTW, nice rifle

If you barrel isn't freefloated than resting the handguards on something will change your poi.  How much??  I don't know, it depends on how much pressure you were putting on the gun.  I am only speaking from experience with my colt govt a2 20".  Almost any pressure will change poi.  I tried using my sling once tightly and if shifted my poi quite a bit to the right and up a bit.  Probably 3-4" inches.  If  your going to use bags.  Try to put them as close to the receiver as possible.  And don't push down as much on the gun.  Just let it rest real nice like.  This is what happens when you don't free float.  It's a bit more touchy.  It may also be different if you had an HBAR.  Some say, they don't flex as much.  

Dang, we must've posted at the same time.

Thanks for the feedback guys. I knew that a non-FF handguard could lead to some POI shift with the thinner barrel profile, but I was surprised at the magnitude of the effect. To be honest, the 12" shift I reported was an estimate based on the fact that, with a hot barrel, I had to aim below a 12" plate to ring it. I will try to get back out to the range and run a more controlled test with paper targets to confirm the true magnitude of the shift.

In and of itself, such a shift is not a huge problem, as its easy to allow for it as the barrel heats up. In reality, at that distance I usually shoot prone with the rifle resting on the magazine... this may reduce the magnitude of the error vs the sandbag rest. Once I have some more accurate data I'll post here again.