POI change when using a sling?


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dmancornell
October 29, 2013, 04:48 PM
Hi, would like some feedback on this issue. I zero all my rifles rested on sandbags. Sometimes I shoot offhand with a sling and I often see a change in POI, sometimes drastic.

Two examples are a AR-15 (govt profile barrel) with a Turner sling, and a Steyr Scout with a Galco safari sling.

- At 100 yards, cross-legged with the sling loop tucked in tight almost up to my armpit, the AR-15 is low and left about 3-4 inches.

- At 200 yards, prone with the sling loop around my tricep, the Steyr Scout misses the A4 paper target entirely.

I go back to sandbags after missing a bunch and POI is back to normal.

My hunch is the pressure I am exerting on the sling is pulling the barrel down and left, and the Steyr Scout being especially impacted due to the thin barrel profile.

Is this expected? How do competitive shooters who use slinged positions deal with this? Or am I using the sling incorrectly?

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wally
October 29, 2013, 04:58 PM
This is why they make free float tubes for ARs. Other rifles glass bed the action and make sure the rest of the stock doesn't touch the barrel.

sirgilligan
October 29, 2013, 05:37 PM
That is not unusual.

Caliper_RWVA
October 29, 2013, 06:08 PM
+1 on free floated barrels, or at least sturdy stocks that won't deflect as much (think military stocks: M1, M1A)

I have a theory that iron sights are less affected by this because you are pulling the front sight along with the muzzle whereas a scope on the receiver will not move when the barrel is deflected by the sling tension. Haven't tested that yet though.

You might try using less sling tension to lessen the effect. Alternately, zero from prone using the sling so the tension is accounted for in the zero. If you are using a sling to shoot, your prone groups ought to be nearly as good as from bags, so why zero off bags anyway?

Competitive shooters using a sling will do several things. First, they often have free floated barrels. Second, many competitors will mark their sling so the sling adjustments are very repeatable and thus anything the sling tension may be doing to the rifle is the same each time and third they zero while shooting slung up and write down their sight adjustments for each position and distance.

Reloadron
October 29, 2013, 06:25 PM
My hunch is the pressure I am exerting on the sling is pulling the barrel down and left, and the Steyr Scout being especially impacted due to the thin barrel profile.

Your hunch is on target and as to the fix? As was mentioned, free float the barrel.

Ron

henschman
October 29, 2013, 07:56 PM
It definitely changes the POI if the rifle isn't free floated... especially if the sling swivel is attached to the barrel, like a factory AR-15. I always zero a rifle the way I'm going to shoot it in the field. I love my free floated ARs with the slings attached to the handguard... always the same POI no matter whether I'm unsupported, slung, solid rested, on shooting sticks, etc.

SlamFire1
October 29, 2013, 08:58 PM
With my match M1 Garands or M1a's, my sitting rapid fire zero has a different elevation from 200 standing and the windage change is a consistent 1L or 2L (half MOA to full MOA Left) from standing.

At three hundred yards my windage is always a 1L or 2L for prone rapid fire. I have no idea what my true 600 yard windage zero as I have never shot in conditions that I consider, true no wind conditions, at 600 yards. I assume it is "0" on the windage gage, but I look at wind conditions, make my best guess, then correct off the two sighters.

I would have to look at my data books, but AR's are different and do not necessarily follow the same windage rule. But, the zero's between standing and sitting at 200 yards are different. Just a couple of clicks, but it is different.

What I have found shooting in small bore prone, is that breaking position will change your point of impact. Small bore prone is unforgiving in this regard and errors in position, trigger pull, sight alignment are not masked by recoil and blast. You will see groups move up and down as you slide your face along the cheekpiece and you will see groups move left or right as the buttplate moves on the shoulder. You will also see groups move based on how you hit the trigger and grip the stock. Basically, everything affects everything.

Welding Rod
October 29, 2013, 09:22 PM
Solution is simple....stay off the bags.

Seriously though, gov profiles are horrible about this POI change. Floating works well, but so does a HBAR. In my experience floating a gov is a little better in consistent POI when going back and forth between no sling and firm sling tension, but a quality HBAR may be more accurate over a wider range of loads / ammo and may still outshoot a floated gov barrel overall when going through the positions if the shooter only uses light sling tension when slung up.

dmancornell
October 29, 2013, 10:43 PM
Thank you all for the help.

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