First time out with 6.8 little disappointed

[ammo]

Had a bit of a disappointing first time out with the Bushmaster 6.8 today. Went over to a friends place and did some target shooting out on his land. It seems to cycle fine though it fails to lock back after the last round. Out of 6 mags, only once did it latch after the final round. I also found that the sights seem to be off, shot placement is about 12 inches high at 60 yards, didn't expect such a drastic miss. Going to prepare to make some changes for the next time out, as well as zero it. I'd like to hear from the forum guru's as to what you may think may be the issue. I have done some searching around the forum and have read about a "short stroke" issue.

Here is some info about the rifle:
1. Fired both Hornady and Remington rounds today.
2. It has the Magpul BAD lever (going to uninstall next time out).
3. Only used the factory Bushmaster magazine (going to purchase a PRI before next time out).
4. I was using a MBUS Rear sight, factory front post.

[jdgilbert]

Remington loads are reportedly weak, not sure about Hornady. What buffer and spring are you using? If you have a standard carbine setup it may be under gassed for that ammo.

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[DB86]

I would say the point of impact was due to the MBUS rear sight. Not that they are bad in any way it's just like installing a scope for the first time... Naturally you will have to do some adjusting to get it zero'd in. It only takes a tiny fraction to make a big difference out at the target.

As for the failure to lock back after last round: where is your brass landing after you shoot? Front of rifle being 12:00 back of rifle being 6:00 and 90 degrees from the ejection port being 3:00 where would you say it ejects? Also what buffer system is the lower you were shooting it on? No mark on buffer or H or HH or H2 etc. it could possibly be the mags, but sounds like maybe under gassed to me. the ejection angle will help to diagnose the problem. If you have a 5.56 mag load 1 6.8mm rd in it and fire it and see if it locks open that should rule the mags in or out before you pay 35 bucks for a PRI. Hope this helps.

[Stack]

I have a couple Bison uppers that won't reliably lock the carrier back with starting loads but with anything beyond the softest loads they lock every time. This is with 2.6 oz buffers and red springs. Hottest loads still drop at 4:00. As for the sights being out a bit sounds normal to me. I mount the upper in a fixture and look through the bore at something (my neighbors AC unit) and twist the sight in. Gets me on paper at 100 yds.

[Equalizer_2]

The Remington loads are on the light side but the Hornady generally work well. Check the top of your mag lips to see if the bolt is dragging on them. The other suggestions are also good to check.

[Clint]

BAD levers are known to cause issues with the bolt catch.

Check if the front site base is "F" marked.



The MBUS are designed for F marked height.

[John A.]

Ammo, all of these are good answers.

[ammo]

Ammo, all of these are good answers.

I agree.

I have taken a look at the items suggested above. Here is what I have found; the the bolt is in fact dragging along a small area on the top of the magazine. Also, there is no "F" on the front sight post.

I have only fired the weapon one afternoon with the BAD lever, and I found it awkward (20 years active duty without the lever, old dog new trick analogy I guess). I am definitely leaning towards removing it. I am also going to look around for another magazine.

On the issue of the non "F" front sight with the MBUS rear sight, will I be able to zero with this set up? Will the front sight adjust up high enough? Or should I replace the MBUS with another sight?

[John A.]

12" high at 60 yards is a lot, but each click of the front sight is going to move it about 3/8" at 25 yards.

And most folks will zero it at 25 yards and then go out to 100 yards and check for zero again to fine tune it.

At 25 yards, you're probably only going to be inch or so high (about 3 clicks on the front sight)

[Clint]

Here it is. A taller sight post to compensate.

http://www.bushmaster.com/products.asp?cat=21


A2 Front Sight Post for Flat-Top Colts
Colt A3 Removable Carry Handle Sights are taller than A1& A2 Rear Sights, and require an .040” taller Front Sight Post. Also for use with Swan Sleeves & ARMS #40 sight.