Last round in the mag POI shift?

kmontang

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In an extended conversation on another forum, a couple of folks are claiming that when shooting AR's, they consistently get a POI shift in the last round fired on a given magazine if the round is also the last round in the mag. This seems unlikely to be a mechanical thing to me, but I wanted to check and see if that sounded plausible to any of you guys?
 

2010Ruger

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Possibly due to the bolt locking rearward after the last round and not reciprocating forward as with all the previous rounds fired. Different recoil movement=different poi.
 

sirhrmechanic

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Possibly due to the bolt locking rearward after the last round and not reciprocating forward as with all the previous rounds fired. Different recoil movement=different poi.
Good theory, but would the bullet not have already left the muzzle before the bolt locked back?

maybe the bolt traveling over the magazine follower vs traveling over a loaded round causes a different harmonic?

Does seem odd. Not saying it isn’t happening. But seems to me like the lock back happens well after the bullet has left the muzzle.

Anyone have slo-Mo of an AR cycling including the bullet leaving the muzzle?

Interesting topic!

Sirhr
 

6.5SH

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I'd think a way for them to test it would be fire a group single loaded.
 

whatsupdoc

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Possibly due to the bolt locking rearward after the last round and not reciprocating forward as with all the previous rounds fired. Different recoil movement=different poi.
The bullet has left the barrel before any of that happens so that is incorrect.
The reason is there is upward pressure on the bolt carrier from the magazine for every round but the last.
Seems not all have this issue.
 

kmontang

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The bullet should be gone before the bolt makes virtually any backward movement, and WELL before lock-back occurs. that should negate any effect of friction of the bolt on bullet vs follower I would think. 1:15 mark for any of you who dont want to listen to Larry

 
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2010Ruger

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Great videos. I nominate anyone besides me to test this phenomenon because my poi with ARs is never where it’s supposed to be...
 

whatsupdoc

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Dont need a video, if the bolt unlocks while the bullet is in the barrel the case head would be unsupported with the barrel/chamber under pressure
and bad things would happen.
 
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MCHOG

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The bullet has left the barrel before any of that happens so that is incorrect.
The reason is there is upward pressure on the bolt carrier from the magazine for every round but the last.
Seems not all have this issue.
This makes the most sense to me. The way to test it would be to use something like the wedge and see if that fixes it.
 

S197

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This is the only thing that I could think of.

A loaded mag might put pressure on the carrier. Once the last round is chambered, there might not be any upward pressure on the carrier.

if this is happing I would start looking at the head spacing on the bolt, and lugs for ware.

you could chamber a blank in the upper and see what kind of movement there is.
 

Molon

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In an extended conversation on another forum, a couple of folks are claiming that when shooting AR's, they consistently get a POI shift in the last round fired on a given magazine if the round is also the last round in the mag. This seems unlikely to be a mechanical thing to me, but I wanted to check and see if that sounded plausible to any of you guys?

Internet Commandos are always looking for an excuse for their lame marksmanship skills. The 10-shot group pictured below was fired from one of my Krieger barreled AR-15s at a distance of 100 yards with only 10 rounds loaded in the magazine. Where's the shift in the point of impact?








.....
 

bfoosh006

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In an extended conversation on another forum, a couple of folks are claiming that when shooting AR's, they consistently get a POI shift in the last round fired on a given magazine if the round is also the last round in the mag. This seems unlikely to be a mechanical thing to me, but I wanted to check and see if that sounded plausible to any of you guys?
Could you share a link to the "other forum" ... I would like to read some of the comments.

The last round POI change idea... is more likely to be someone psyching them selves out. Lol
 

MCHOG

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Internet Commandos are always looking for an excuse for their lame marksmanship skills. The 10-shot group pictured below was fired from one of my Krieger barreled AR-15s at a distance of 100 yards with only 10 rounds loaded in the magazine. Where's the shift in the point of impact?







.....
Screen Shot 2020-09-17 at 4.08.56 PM.png

Just kidding. Nice group.
 
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nn8734

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Ive been in the middle of load testing for my mod H and Mk11 lately. Firing 5 and 10 round groups at paper and steel. Have not noticed any POI shift on the last round in the mag for either weapon.
 

lowlight

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    Doesn't have to be locked back, but it clearly starts to move while the bullet is still in the barrel, and gas travels much faster than the bullet

    It's really dependent on the rifle, Small Frame vs Large frame, you don't see in a 556 but you will see it in a 308, there are huge differences in shooting these two systems "well" as they are different in how they work in terms of recoil, recoil management and when the bullet is actually gone.

    ARs have 3x the lock time of a bolt action rifle, so fundamentally you have to be perfect compared to a bolt gun. The hammer has to travel up and around strike the firing pin and then the machine begins it's job to operate. That movement has an effect if the shooter is not sound.

    We certainly can, and do beat the bullet in terms of our movement before the bullet leaves the barrel and the carrier is certainly moving backward. The heavier carrier and buffer of the large frame can and will move things around.

    A 20MOA base is only .11" different from front to back, a thousand of an inch is more than 1 MOA almost 1.75 MOA at 100 yards. So it's beyond the system it's into both our shooting and the system.

    It also depends on where the gas block is in terms of the barrel length, the bullet weight and speed as well as the trigger system.

    The system is moving it might not be locked back but it is in motion.
     
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    xtacleone

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    Doesn't seem make sense to blame the mag. Once the trigger has bene pulled and round has been fired, the magazine will have nothing to do with impact. I would say they are anticipating, the shot which affects shot placement.

    UNLESS, the spring is fatigued, follower misaligned, etc and the round is not fully seating/bolt is back a few thousandths of an inch. This could in theory change how the round is headspaced in respect to the previous shots fired.
     
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    rustyinbend

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    Interesting debate ... seems to me that given where the bullet is in the cycle of fire, "if" this is real, the "only" thing it could be would have to take place around the difference between upward pressure on the BCG from a round in the magazine (more upward pressure), vs. the upward pressure on the BCG from no round and just the magazine follower (less or no upward pressure). And the only way that happens is if the BCG's channel or chamber are not a tight fit and within specs. Blaming anything after the bullet leaves the cartridge ... would be difficult to visualize since the bullet is gone before anything material happens. I have a lot of AR's, and I've never seen a "last shot" inaccuracy issue. I'll watch more closely for it in the future.
     

    rth1800

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    I would not call it "inaccurate." It is a very consistent shift, normally lower shot. It will actually form it's own little group if you fire a few mags at same target.
     

    Cheyenne Bodie

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    Sounds like the forces acting on the carrier are different on last round, effecting POI. At least that's what I'm picking up from what's going down in here.
     

    Silverjay

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    I have run several test on this with my Larue OBR. Ran groups single feeding and no fliers, ran groups with +1 in the mag and no fliers, ran groups with locking back on the last round and had some. The more I worked on follow through and setup the less last round fliers I had. Best I could tell the dynamics were slightly different and exploiting MY weaknesses.
     

    xtacleone

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    Interesting debate ... seems to me that given where the bullet is in the cycle of fire, "if" this is real, the "only" thing it could be would have to take place around the difference between upward pressure on the BCG from a round in the magazine (more upward pressure), vs. the upward pressure on the BCG from no round and just the magazine follower (less or no upward pressure). And the only way that happens is if the BCG's channel or chamber are not a tight fit and within specs. Blaming anything after the bullet leaves the cartridge ... would be difficult to visualize since the bullet is gone before anything material happens. I have a lot of AR's, and I've never seen a "last shot" inaccuracy issue. I'll watch more closely for it in the future.
    I like the way you are thinking about this, makes sense . But I want to circle back to the idea of the bolt not fully seating, the chamber could be too tight and the last round might not seat as deep, it still go into battery. The effect could be that the bullet would have a bigger jumpy to get to the rifling, maybe just a few thousandths of an inch, but could be enough to affect accuracy.

    Remember, there is a forward assist on ARs for a reason. I know rifles and ammo are better from the Stoner inception. But I would be curious if utilizing the forward assist on the last round, to ensure the tightest seating of the round, has any impact.

    Also, check the brass to see if if ther is more of a burn around the neck of the case, see if there are any scratches or markings on the base of the case AND last but not least ask your friend if it feels like they are getting more gas in the face on the last shot.
     

    Culpeper

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    Internet Commandos are always looking for an excuse for their lame marksmanship skills. The 10-shot group pictured below was fired from one of my Krieger barreled AR-15s at a distance of 100 yards with only 10 rounds loaded in the magazine. Where's the shift in the point of impact?







    .....

    Fake news
     

    xtacleone

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    Internet Commandos are always looking for an excuse for their lame marksmanship skills. The 10-shot group pictured below was fired from one of my Krieger barreled AR-15s at a distance of 100 yards with only 10 rounds loaded in the magazine. Where's the shift in the point of impact?
    .....
    oh yeah Molon? Look at my 90rnd group fired from my custom Highpoint 6" barreled AR15s at 1200yards from a moving donkey. Only one round in the magazine. I hand loaded each shot with the same bullet and same brass. Who's the internet commando now?
     

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    kmontang

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    Doesn't have to be locked back, but it clearly starts to move while the bullet is still in the barrel, and gas travels much faster than the bullet

    It's really dependent on the rifle, Small Frame vs Large frame, you don't see in a 556 but you will see it in a 308, there are huge differences in shooting these two systems "well" as they are different in how they work in terms of recoil, recoil management and when the bullet is actually gone.

    ARs have 3x the lock time of a bolt action rifle, so fundamentally you have to be perfect compared to a bolt gun. The hammer has to travel up and around strike the firing pin and then the machine begins it's job to operate. That movement has an effect if the shooter is not sound.

    We certainly can, and do beat the bullet in terms of our movement before the bullet leaves the barrel and the carrier is certainly moving backward. The heavier carrier and buffer of the large frame can and will move things around.

    A 20MOA base is only .11" different from front to back, a thousand of an inch is more than 1 MOA almost 1.75 MOA at 100 yards. So it's beyond the system it's into both our shooting and the system.

    It also depends on where the gas block is in terms of the barrel length, the bullet weight and speed as well as the trigger system.

    The system is moving it might not be locked back but it is in motion.
    But isn't that initial movement you are referencing the same from round to round within a mag? and if its the same round to round, shouldn't it have a net-zero affect on accuracy from round to round? Thats the point being made on the other thread-that the last round in a mag is inherently less accurate