Greetings,
I have been scratching my head about that. it happened, I made it happen again and again and again. At first I thought I kind of screwed up at the KD.
Shooting prone loop sling and shooting prone loop sling with hand guard resting on sandbag.
Rifle : CAR-15 RRA mounted with ACOG TAF11
Distance 400 yards
Shooting loop sling POI = 0"
Shooting loop sling + sand bag POI = -11"
I am 100% sure my sight picture is the same, my position is the same, my trigger control is the same, I shoot rifleman cadence. Only difference is the hand guard support removing some wiggling. I did follow up on each shoot. The group spread is average 12" with most shoots within 8"
I revised my last year notes and same thing happened but I did not pay too much attention because my goal was to get the come-ups on another rifle and I did not care not to re qualify.
a) I think it is not normal such a difference between support and non support? What am I doing wrong?
b) Any of you ever saw that too and do you address it with 2 sets of come-ups?
Thank you
Pull lower off and grasp handgaurd with one hand and upper receiver with the other - twist. This may be a loose barrel issue. Maybe not, but always worth checking.
Where is the sling attached? Is it bending the barrel slightly? Recoil in the two positions could also be causing a difference in POIs.
No loose barrel.
Sling attached on bottom sling attachment standard one on the rifle.
You never use the sling while resting the rifle on the sandbag. You can use the sling with the sandbag acting to support your forearm or wrist.
If you were getting barrel deflection the behavior that you describe, sling and sandbag lower than sling alone, is what I would expect since the sandbag is keeping the barrel from moving lower.
But you should see the same behavior at 100, 200, 300 or even 25 yards to a lesser extent.
Assuming your barrel isn't floated (floated barrel doesn't appear to be standard on the car15) your answer is simple sling tension deflecting the barrel. An 11 change in poi is only 2.75 MOA and you can easily get that much poi shift with sling tension on an unfloated barrel. Add a float tube and I expect your problem will go away. Same thing will happen at every distance but slow prone is where it's most noticeable since that's usually where you'll shoot the smallest groups.
Nothing new to me here.
I've known for many years you get a different zero "supported" than "unsupported" with the AR platform.
And, that is what you have here.
FWIW,
Vintage Sniper match for the CMP Games, match gives two options. Use of sling, or use of SandBag. Never both sling and sandbag.
Shooting prone off sandbag, I do not do the Olympic X as we teach in Appleseed (drawn up trigger knee). I lay DIRECTLY BEHIND the rifle (this is very important), and stretch my legs directly out behind me, feet just over shoulder width apart. Feet flat on the deck also.
Support Hand, off sandbag, grabs the rear sling swivel (as a rear bag is not allowed), and making a fist I support rifle in my shoulder. NO HANDS whatsoever on the handguard. That will affect your POI.
I found both positions to be accurate, but the sandbag without sling was definitely more accurate for me.
The correct names for the positions are: (with sling) Unsupported Prone, (Sandbag or Bipod), Supported Prone.
Here is a cool page that describes supported prone:
http://8541tactical.com/2013/09/14/the-supported-prone-position/
R/
Chris
R/
Chris