Today I was at the range, and was sometimes using a 30rd Pmag, while other times I was using one of those “Bob Sled” mag-shaped devices that you insert in the mag well to single-load rounds in an AR.
When I used the Pmag, all went well and my point-of-impact (POI) was what I expected. However, when using the Bob Sled device to single load rounds, even though using the same ammo (Black Hills 69gr match ammo), my POI was WAY off (it was about 4" to 6" low).
Would the Bob Sled be a likely cause of such a dramatic shift in POI? If so, is there anything I could do to fix this (assuming I still want to use the Bob Sled), apart from having to re-zero the rifle for this device?
Agreed, it shouldn’t. I cannot imagine why the round, once stripped off the top of the mag and seated in the chamber, should care whether it was loaded from a Bob Sled or a Pmag.
Barrel is free floated in a Troy quad rail (middy 9"), shooting off sandbags. Same position, same ammo, with/without Bob Sled. I know, shouldn’t make a difference. But I was getting tight groups with over 50 rounds through each scenario, and the POI difference was obvious. I guess it has to be something else, but I cannot figure out what it would be.
Was the magazine touching the ground or bench? If so, that could explain it, pivoting up in recoil with the magazine, leading to higher POI. Just my guess…
I thought that there could be a bit of a difference, but the first round out of a 30rnd and the sled should be the same. Put the sled in the same thirty (most sleds I’ve seen are in 20rnd mags).
Very odd, I’m stumped. If the round is loaded from the sled and damaged such as set back, I would think it would be higher since the pressure should be greater. I’ve only seen a shift on a single load or from the first round feed, but it was really small(1/4")
I would double check evrything, make sure you aren’t influencing the barrel when using the 30 round mag or shifting point of aim, etc.
Did you zero with that one mag? if so-check it with a standard GI mag-if that changes impact-I have a Doctor Madblood theory
Do you have a compensator on it? maybe the down force from the comp is causing a little barrel dip with the light (sans magazine) rifle. Seems wacky, but I’m otherwise stumped.
Thanks for all the suggestions. To answer some earlier questions, the barrel is Sabre 4150 CMV steel, and no there was no optic involved yesterday, I used an LMT detachable rear and a fixed front sight. Also, I did not zero specifically with either of these mags. I had zeroed the rifle previously with a USGI mag and the attached sights my last time out, and it shot fine this time as well, when using the Pmag. It was when I had the Bob Sled inserted that I kept getting unpredictable results. To be clear, I don’t know for sure that the Bob Sled was CAUSING the lower POI, but only that the lower POI occurred WHEN the Bob Sled was inserted.
I thought last night while I was cleaning whether I had done anything else to the rifle recently, or if I did something different while using either the Pmag or the Bob Sled. But the only thing that came to mind was that I recently added a VFG (but that was on there and in use while firing in BOTH scenarios).
I’ll try some of these suggestions next time out. Thanks!
Sitting here thinking the big difference in feeding will be resistance vs allot less resistance. The carrier stripping a round off the mag will be slowed down by the process. When it just pushes a loose round on the bobsled the carrier doesnt slow as much so the round is pushed into the chamber faster and also comes to a more abrupt stop. That could be just enough to push the bullet further out of the case (lower pressures unless its on the lands).
If you have some calipers handy and still have some of the ammo, load a round stripped from the magazine and extract it by pulling back on the charging handle while pushing the case to keep it centered and not rubbing the chanber wall. By keeping the loaded round off the chamber wall duriing extraction you will keep most other variables from damaging the loaded round. Then do the same with your single shot follower. See if the latter is longer and be sure to measure before and after as i would be really surprised if they were all loaded exactly the same length from tip to case base. The ogive may be a better way to measure them if you have the tool.
Again thats just an idea i came up with so take it how you will.