last round fired is always off

[corey4]

i just got into semi autos not too log ago.

all shoots fired for the sake of this discussion are from the bench with a rear bag.

i noticed, whether i am shooting my mk18 SBR or my scar 17, the last round fired when the rifle locks the bolt back is always off. on my scar 17, is't usually .5 mil low. on my SBR, it's been a few months and i forget which direction, but the last round is always off as well.

is it me? or is this just how it is, the rifle isn't competing the cycle?

attached is a pic from yesterday with the scar 17. these shots were fired in semi rapid succession. meaning; fire, ride the recoil, let it settle, fire. no time in between for breathing.

300yds
scar 17
168 FGMM
trijicon accupower set on 6x
14 rounds total believe it or not
6" pie plate

as you can see, i have 13 in the plate, but one way the fuck out there, which i am assuming is my last round (it's a little tough to see 30cal holes at 300yds on 6x). a little back ground info, when i shoot the steel plates and take my time, i see the rounds appear on the steel and my last shot is always low. this is why i am assuming the round way the fuck out there was my last round.

 

[Va_Gentleman]

If it is the last round and your BCG locks back; sometimes the last pill out of the barrel will be different from the group.

That has always been the common consensus among my circle of buds. I have never really seen much of a variance on the last round in the mag with BCG lock, but some feel that it can affect the last shot.

 

[waveslayer]

I have had the same problem. I have tested this myself several times, the bolt locking changes my semi automatic barrel vibration. Thus throwing off my last shot. Now when I shoot, if I'm going for a 5 round group, I load 6 bullets. That has worked for me.

 

[jhr1986]

I haven’t noticed this when I shoot my ar15s for groups, which admittedly is not terribly often. I would think the bullet would exit the barrell before the BCG locks to the rear therefore not affecting POI.

 

[corey4]

I have had the same problem. I have tested this myself several times, the bolt locking changes my semi automatic barrel vibration. Thus throwing off my last shot. Now when I shoot, if I'm going for a 5 round group, I load 6 bullets. That has worked for me.

that what i have been doing, or ill just hold over .5 mil and make the hit.

 

[corey4]

I haven’t noticed this when I shoot my ar15s for groups, which admittedly is not terribly often. I would think the bullet would exit the barrell before the BCG locks to the rear therefore not affecting POI.

you would think that, but it's the same concept as breaking form when shooting a bolt gun. like when you take your finger off the trigger before the recoil impulse is over, or taking your head off the stock before the recoil impulse is over.

 

[jhr1986]

you would think that, but it's the same concept as breaking form when shooting a bolt gun. like when you take your finger off the trigger before the recoil impulse is over, or taking your head off the stock before the recoil impulse is over.

That is only partially true; yes if you lift your head off the stock within milliseconds of breaking the trigger or slap the trigger your can affect the shot because you disturb the gun while the bullet is still in the barrel. Once the bullet has exited the barrel, nothing you do to the gun can physically affect the bullet. Therefore the question is whether the bullet is still in the barrel when the bolt locks back. I don’t expect that it is, but I could be wrong.

 

[TheGerman]

Known phenomena.

I remember reading something about David Tubb talking about this exact thing. Not sure if the cause was ever totally proven, but I've seen it as well.

I always wonder if its something with the bolt getting locked back and the 'stop' in the bolt carrier group, or if its how the last round feeds out of the mag differently because all subsequent rounds are coming off the top of a loaded cartridge and the last round is coming off the follower.

 

[whatsupdoc]

The bullet has left the barrel and there is zero pressure in the barrel when the bolt is unlocked, it then must travel rearward to the stop and start to return interrupted by the bolt stop.

Do not doubt the condition exists but the event would happen while the bullet is traveling down the barrel.

 

[redneckbmxer24]

I’ve never experience this and don’t see what difference the bolt locking back or not could make.

The bullet exits the barrel before the carrier has even moved enough to bring the bolt out of battery.

 

[C_R_Slacker]

I see this sometimes. Never could figure out if it was me or something actually happening with the rifle.

 

[corey4]

sometimes i don't know if i'm going to run dry. let's say i load 8 rounds for whatever reason. i shoot 2 or 3 at 100, then one or 2 at 300, then someone calls cease fire to go down range. when i commence to back to shooting, maybe i checked my ammo, maybe i didn't feel like it. or maybe i loaded a mag with 6, i shoot the shit with someone for a bit, or did i load 5? or 8? hell i dunno, im just gonna shoot.

 

[bigjake83]

i just got into semi autos not too log ago.



i noticed, whether i am shooting my mk18 SBR or my scar 17, the last round fired when the rifle locks the bolt back is always off. on my scar 17, is't usually .5 mil low.



It's You.

 

[corey4]

anything in particular that i need to work during my follow thru?

 

[XLR308]

With my 24" bull barreled AR all I ever use is a 5 round magazine and have never noticed any shift in my groups that weren't directly induced by me.
The lighter shorter barreled rifles I have will definitely tattle tale on me if there is any inconsistency in my form or trigger pull.
That has been my experience and have allways considered inconsistencies in my groups with an AR to be shooter error on my part.
Me and my brother both tested this theory with a lead sled as he was having issues with flyers, he concluded as well that he was the loose nut in the machinery.

 

[samnev]

I've experience the same thing when my first 4 shots can be covered by a dime 50% of the time the 5th shot is usually 0.5 moa low. I chalk it up to buck fever as I can feel the tension in my body increase before the last shot. This happens when shooting either my AIAW or one of my 308 or 556 semi AR's.

 

[Lowlight]

Tension on the bolt from the magazine,

a Scar is not a precision rifle, the full mag pushes on the bolt, when it empties the bolt no longer has the tension on it, guy would usually load an extra round or not shoot them all

Last round fliers are common in certain semi autos

 

[bigjake83]

anything in particular that i need to work during my follow thru?

XLR308 hit the nail on the head....

Due to the lack of a 3rd Recoil impulse, and the happy trigger finger (it's what I call it).

Many times I'll see people do this, with bolt guns and Semi's, they will put 4rds in one hole, and when they know they're on their last round the almost psych themselves out. And this will cause them to white-knuckle the grip, jerk the trigger, and Cant the rifle.

This is where I would start....

Have your range buddy load your mags for you so you never know what you're round count is.

Put a anti-cant device on all of your scopes, and make sure they are perfectly level with the scope and your action.

If you start to feel fatigued just pause for a moment and relax, don't try to rush to get that last round in. (BUT.. don't take too long if you know your chamber is hot, it will start cooking that round and you'll get vertical stringing)

AND TO MY BIGGEST PET PEEVE!

If you go to a range like I do and there's no RSO on duty and you have some Jackass calling a Ceasefire every 5 fucking minutes because he can't see his groups, tell him to Stand the fuck by until you're done.

This happens to me at least once a week mostly from Hunters. Or people who don't know proper range etiquette.

 

[redneckbmxer24]

Tension on the bolt from the magazine,

a Scar is not a precision rifle, the full mag pushes on the bolt, when it empties the bolt no longer has the tension on it, guy would usually load an extra round or not shoot them all

Last round fliers are common in certain semi autos

Holy hell. That thing must have some terrible fitment if that little bit of difference in carrier tilt makes that much of a difference.

In AR-15's the accuracy increase from addressing carrier tilt either with one of the carriers that help it like JP or installing pads on the rear of the carrier is much smaller than even a great shooter can really see.

 

[Lowlight]

A 20MOA base (I know but you get the point )

.45 - .375 = .075"

That gives you 20 inches at 100 yards if you break that down into 1 MOA you get

.075 / 20 = .00375"

.00375" of error is a minute at 100 yards

 

[WarbirdWeaponry]

That is an excellent comparison, thank you!