Problem with FN-49 after installing new receiver cover - disintegrating bullets

[Kabal]

My Dad recently purchased an Egyptian contract FN-49 in 8x57 IS which we both enjoy shooting.

Before today's shooting session, we had fired around 150 rounds through it, mostly S&B 180 gr. FMJ. We carefully adjusted the gas system when he got the rifle, and we never had any problems with the gun.

My Dad wanted to put a scope on the rifle, so he bought an FN-49 receiver cover on the Internet that has a picatinny rail installed. He attached a Burris P.E.P.R. mount and a Nikon Fieldmaster scope.
The idea was to be able to switch between iron sights and the scope by switching receiver covers.

Today we wanted to try out the new setup and sight in the scope at 100m, and something very unusual happened:

I fired my first couple of rounds (S&B 180 gr. FMJ), and when I checked the target for hits, I noticed several strangely-shaped holes at the very bottom of the paper, but nothing that looked like it was made by an 8x57 bullet.
I wondered if those holes had been there before, or if there was something wrong with the ammo.

On an impulse, I removed the receiver cover with the scope and re-attached the original one with iron sights. I fired around 10 shots (some S&B, some reloaded stuff), which punched nice, clean holes in the paper target.

I then switched back to the receiver cover with the scope and put up a new paper target. I aimed a little higher and fired another round (S&B). Instead of a clean hole, I got a paper target that was plastered with many holes of different sizes:

We immediately stopped shooting.
Apparently, the bullet had broken apart in flight. The same thing must have happened when I fired my first few rounds (which were aimed lower, so most of the shrapnel didn't hit the target).

For some bizarre reason, this seems to happen only with the new receiver cover attached (the one with the scope).
We've used the same recoil spring with both receiver covers. The ejected cases are in good condition, even those that belong to the bullets which disintegrated in flight.

Can you guys think of an explanation for this? How is it possible that this problem only occurs with the new receiver cover?


I've uploaded some pictures of both receiver covers:

 

[rcmodel]

I believe the scope is so far off, you are hitting the ground in front of the target and peppering it with bullet frags and rocks.

There is no possible way changing the receiver cover could cause bullets to key-hole or come apart in flight.

rc

 

[Kabal]

Thanks, that makes a lot of sense!

We used an in-chamber laser bore sighter to roughly adjust the scope before shooting. The bolt didn't close on it, so I didn't expect much accuracy. I didn't expect the scope to be this far off, though.

 

[Edarnold]

If your receiver has the mounting dovetail on the left side, a mount like the FN original or the Echo would place the scope much lower, but still clear the iron sights if the scope objective isn't too large. The rig you have right now looks awfully cumbersome to shoot.
Try searching 'FN 49 scope mount' for more information.

 

[rcmodel]

There is no good to come of mounting a scope on a relatively rattily loose receiver cover in the first place!

rc

 

[barnbwt]

So, as we can see, shooting at a bad guy's feet does more than merely "make him dance"

That rail mount is pretty well done (easy to do, since the top cover's a giant steel block on the FN49 ), where'd you find that? With low profile rings and a lace-on cheek pad, that would be a very slick setup, with no permanent modification. I have a Lux 30-06, and while the peep sights are very nice, there are times when you want to scope your gun.

I'm pretty sure the top covers would interchange between the different models, but does anyone know to the contrary? I'll see if Numrich happens to have any (wish me luck guys, I hope I don't run out of food before I find it on their crummy search engine )

RC, I just dug mine out of the safe for a sanity check (since top covers are typically mounted loosely) but there is nothing flimsy or rattly about that thing whatsoever. The rear sight's on there by design, which is probably why FNH had to secure it so well in the first place. It sure beats drilling/tapping the receiver of a gun this nice! I suppose the railed cover could be drilled/tapped for a few set screws to really secure it to the receiver, as well (though takedown would be more annoying)

"The idea was to be able to switch between iron sights and the scope by switching receiver covers."
You will likely find that to be too inaccurate to take advantage of without a re-zero each time (not a big deal, but not something you can do on the fly, either). But try it out though and get back with us; if it's repeatable within a few MOA (the guns expected accuracy), that'd actually be a pretty cool capability for such an "obsolete" rifle .

TCB