The gun moves under recoil before the bullet even leaves the muzzle. Especially a short/light 6.8. If the gun moves the same way each time, you can still get good accuracy. The brake will reduce recoil and muzzle rise, so the bore is lower when the bullet leaves the barrel, hence it will shoot lower.

Most dramatic example I have seen of this somewhat unintuitive phenomenon is going from a 180gr JHP to 300gr JHP in my .44 mag pistol. A heavier bullet drops more right? True, but the slower bullet is in the barrel longer while the barrel is climbing up from muzzle flip. Result? At about 25 yds, the 300gr JHP's shoot about TEN INCHES high.

For a long time I thought the bullet would go wherever I was aiming when I pulled the trigger with a rifle. It's just not that simple. To see this easily, change the way you hold the gun at the range with a light 16" .223 or 6.8 from barely touching it to a firm grip, and you will see a significant POI shift. Hold it different hunting than you do at the range...yup, POI shift.

For grins, put a swivel stud on the bottom of the hand guard just in front of your front rest so that the stud rides over the rest under recoil and watch your groups go to hell. There is a reason bench rest shooters shoot big heavy guns with a wide flat bottom.

If your rubber butt pad catches on your rear rest some times, you will see some weird things happen with your groups from that too. Collapsable stocks are notorious for this. One reason I like the ACR is it has a nice long enough flat spot to ride a bag smoothly. I have a SOPMOD I really like, but I have to keep it away from any kind of rear rest or groups open up.

 

OdDuMet, Well it was held the same, same box of ammo and when sighting in we always do it the same way for consistency(not new to that rodeo, we try to take the variables out). The new brake was the only difference and as you can see it grouped well with the scope readjusted.

 

The holding thing is a separate but similar issue. Doesnt apply to what you asked directly. Sorry, I just got a little carried away!

The first paragraph addresses what happened to you....

"The gun moves under recoil before the bullet even leaves the muzzle. Especially a short/light 6.8. .... The brake will reduce recoil and muzzle rise, so the bore is lower when the bullet leaves the barrel, hence it will shoot lower."

 

"The gun moves under recoil before the bullet even leaves the muzzle. Especially a short/light 6.8. .... The brake will reduce recoil and muzzle rise, so the bore is lower when the bullet leaves the barrel, hence it will shoot lower."

It had a brake on before, just a different one, I have never seen that much difference in brakes and the POI, sorry if I was not clear.

 

Did you torque the new brake exactly the same as the old one? Same crush washer? Did one or the other require it be clocked?

This might just prove the old adage that producing accurate rifles is part science and part witch craft.

 

If I'm reading the OP right, the orig was an A2 flash hider? If so, the A2 doesn't do anything to reduce recoil/muzzle rise. It just helps eliminate flash.

 

Good point on the torque, The A2 was down tight, the YHM had to be indexed and so is not as torqued, could not get it around another revolution. Don't get me wrong it is tight but not like the A2 was. Yes same crush washer.

Another good point, The new one is a combo where the A2 is just a flash hider!

"The gun moves under recoil before the bullet even leaves the muzzle. Especially a short/light 6.8. .... The brake will reduce recoil and muzzle rise, so the bore is lower when the bullet leaves the barrel, hence it will shoot lower."

I finally "got it"! Thanks

 

The bullet has to leave the barrel in order for the brake or the compensator to affect the recoil or rise of the barrel. basically what your saying cant be true.

 

Well everything I said about muzzle rise from recoil effect on POI is true, but you are right in that a brake shouldn't do anything until the bullet actually leaves the muzzle, so it's reduction in muzzle rise shouldn't actually effect POI at all...since the bullet has already left the barrel when it starts doing it's thing.

 

Recoil starts as soon as the cartridge fires. It just happens so fast that you don't realize it is going on until you use a brake. Also the higher the velocity/gas speed, the more effective the brake is. This is why brakes work so well with lighter bullets.

 

Barrel harmonics were changed due to different weight of muzzel device. Remember the Browning weight adjustment ring, I forget what it was called, used to fine tune groups for different loads?

 

It was called the BOSS. The difference in point of impact is caused by changing barrel harmonics because of different weight as previously stated. Minor differences in barrel length also make a difference. Anyone that has played with cartridge overall length in an accurate rifle will notice different impact points simply from changing bullet seating depth. Even different lots of the same factory ammo change impact points. Not to say that the difference in recoil doesn't change POA, but other thing are involved also.

 

If altering the crown on a rifle barrel has significant effect on its accuracy, I'm not surprised that the addition/change of a muzzle brake will have an effect, too.

In your case, not only did the POI change, but your groups improved, too. I can't claim to understand all the physics involved, but it makes sense to me.

 

Yes all of the above are true (harmonics/crown/weight) but the A2 flash hider is only ported on top and sides to reduce a dust signature while shooting prone. Gasses disperse differently when using different muzzle devices ie; flash hiders/muzzle breaks/suppressors. POI with these changes are normal and expected. Choose one, sight it in, and call it a day. Many more factors can be added as well but you get the drift. Just my .02 for what its worth! Been aroun BR's for a little bit.
ONE RAGGED HOLE!

 

My groupings stayed the same, which I am happy about, small adjustment made but I was just surprised to get that far a difference. The A2 does not require indexing but the YHM did, all done all through. Thanks Guys!

 

It has a harmonic component. We tune some guns by moving the barrel weight/brake in and out ala Browning BOSS. The same thing happened when you changed brakes. The weight distribution is different and the porting changes things. It is just a matter of physics and there is absolutely nothing we can do other than sight it in with the new gear in place.

Greg

 

Thanks Greg, that was my conclusion too. I originally put this up for the folks that told me I did NOT have to re-sight in after changing brakes, I hope it is a learned lesson, it certainly was for me. Thanks everyone!

 

The Stag upper comes with an A2, not cool enough...LOL The YHM was one of the few made for the Stag, bastard threading 1/2x36 and it was cool! Advertised as a brake and flash suppressor, NO I can tell no difference in daylight but did I mention it looks cool? I always re-sight periodically just for the confidence factor if nothing else since all I get at the lease is 1 shot 1 hog.

 

The Stag upper comes with an A2, not cool enough...LOL The YHM was one of the few made for the Stag, bastard threading 1/2x36 and it was cool! Advertised as a brake and flash suppressor, NO I can tell no difference in daylight but did I mention it looks cool? I always re-sight periodically just for the confidence factor if nothing else since all I get at the lease is 1 shot 1 hog.

All you need to do with the hogs is to wait until they are lined up just right....

Smitty

 

Oh great, well always an expert after the fact and I screwed up...LOL Thanks I will check!