CMP Special in 308 - Range Report - Ammo - and Question

[KnowFear]

Had a fun day getting comfortable with my baby!




Note lower rail and hand-stop with sling attachment point. I have a left hook, and need a stop both for safety, and so that Mr. Hook doesn't slide around uselessly. It also keeps the wood from getting too severely gouged and scratched. You can see the aluminum getting worn off the contact area from just a few hundred shots...

Here is the target - the circles are 1 inch OD:



Anyway, am pretty happy with the groupings at 25 yds, not that there isn't room for improvement. Still, top left and right are bench/sand bag, and lower left is on -awk- a bipod. All in the 3 to 3 1/2 MOA range.

Lower right is prone with loop sling (8 shots)- I am reasonably happy with 4 MOA there, and am really happy with how the rifle comes right back down on target after squeezing one off. All that crap I have attached seems to be working out well with my position and sling tension in regard to NPOA and follow ups. Note that first 4 were slow fire, then did a practice (and painfully slow) re-load, and last 4 were rapid fire (if you want to call every 4 or 5 seconds "rapid").

It was nice to see that the CMP special in 308 loved anything I shoved trough it:
Surplus ball in upper left (6 shots)
Cheap Silver Bear soft points in upper right (8 shots)
Federal Gold Medal Match 168 gr in lower left (4 shots)
And back to surplus ball in lower left for sling/prone (8 shots)

So, here is my QUESTION - my guess is that the tension on the sling, cheek-rest force, etc., is responsible for the shift in POI from bench/bipod to sling/prone. Is there something I can do so that POI is the same for bench/bipod and sling/prone? Or is this what normally happens, and we just zero differently between the two supports?

Note that I did some quick tests this morning to make sure I was not seeing some sort of scope or thermal drift yesterday - I got the same results with sling/prone shifted off to the left, sand bag dead center...


(My Marlin 22 did the same thing while prepping for my first AppleSeed - the solution was floating the barrel, not something I imagine is do-able for the M1 Garand.)


Also posted on CMP sight - I hope the purist's hair doesn't catch fire!

[AFTERMATH]

Great way to IAO.

If you think sling tension might be a culprit - try loosening it up a bit...  It should go away.

The shift in POI is normal and not necessarily dependent upon external forces on the rifle.  In fact, most people will notice a POI shift between the various field positions.  Generally it has less to do with sling tension and more to do with cheek/eye location. 
To put it simply, your cheek weld and eye-sight alignment is different causing you to sight differently.  This is the same reason two people can have two different zeros on the same rifle.
While this affects just about everybody to some degree - it does affect some people more than others.
To, minimize the effects of this - When shooting off a rest/bags, mimic your prone position as much as possible.
A perhaps, better alternative to eliminate this issue would be to just zero in the prone position.

But while this is all true, it does not rule out firing line errors, that might only show up when you're in the prone position
Do you have your shot group analysis sheet handy?  'Cause if it's not a position based POI shift, your group is screaming something else.  You called your shots, right?

[KnowFear]

Yes - all shots called.

Payed particular attention to "DON'T DRAG WOOD", hearing Nero's words reverberate through my head

Checked NPOA with the "eyes closed - breath in - breath out - eyes open test" for each shot during slow-fire 4 shots of sling/prone.

There was definitely a difference, now that you mention it, in cheek position and pressure between the two.  I was very light on the cheek pressure on the bench, and quite heavy on the cheek pressure for sling/prone (my stooopid neck is very inflexible, hence the giant cheek rest...).

[jmdavis]

I think that Aftermath is dead on. The issue is likely more one of head/eye position than one of sling tension. His solution is also the one that I use. Bench and bag are great for developing and testing potential loads. They are less valuable for zeroing a rifle that will be shot in various positions.

Even with a prone zero you may find that standing and sitting zero's vary by up to 1 moa or so due to head position or cant. But this is all good practice.

My next bit of advice if you plan to shoot an AQT with this rifle is to zero in prone on the 400 yard targets of the AQT. Since you are using a scope you should shoot point of aim/point of impact (ie a center hold) rather than 6 o'clock. If you zero on the 400 yard targets you will be all set as far as zero goes for the AQT.

Finally, get or make some dummy rounds and get used to reloading for rapid fire stages. There is no need to buy special clips you can either use the crossed rounds method or simply drop in the clip and pop in two rounds. The real key is to be able to get the reload done smoothly from position with minimal movement.

Good luck,

Mike

[Nero]

+1 to AFTERMATH and jmdavis.  The shift in zero is normal when going from bench to prone/sling, or from prone/supported to prone/sling, for that matter.  It's that subtle change in cheek weld, pressure and therefore sight alignment.  Work out the shift in sight alignment corresponding to 4MOA at your sight radius.  It's appallingly small - one of the reasons we don't spell it out for beginners.   

Which scout scope did you get?  I finally gave up on iron sights for my M1A and went to a forward scope.  I don't need to pay 50+ cents per bang to find out I have lousy eyesight, I can do that for 5 cents (10 cents? now) per bang with my 10/22.

[KnowFear]

I am using the Ultimak mount with the Burris Trophy 2-7x32 with ballistic plex reticle.  It's around $300.

If I had it to do again, I might just go with the inexpensive Simmons 4x handgun scope - I am using that scope for an SKS Scout Scope mount I am developing, and it has been a real work horse.  It's kinda hard to recommend an $80 scope, but so far both of the units I have are great.

(SKScout Scope Mount project, with AppleSeed Shout-Outs here - http://www.sksboards.com/smf/index.php?topic=121061.0 and http://www.sksboards.com/smf/index.php?topic=121949.0 - it's interesting to note that the SKS doesn't suffer much from this shift in POI...)

[Nero]

I've got the Ultimak mount with a Hi-Lux LER on my M1A.  Seems to be doing OK, but I haven't had time for a serious range session since repairing a broken extractor.  At any rate, I can now see what I'm aiming at, which should help, you'd think.   

[jmdavis]

The Burris is a great scope. I have the 2.75x on a Scout that has worked great. The one issue that I have with the entire forward scope concept is that you lose some shooting light even with good optics. With the M1 you don't really have much choice, but it is good to be aware of.

I have successfully used the fixed Burris at ranges out to 460 yards for KD  through a combination of holdovers and adjustment. The ballistic plex reticle is an excellent choice. If you can afford good glass, it always pays to have it.

[Mike Nollett]

That's a very practical looking set up. How did you attach the sling mount/handstop rail to the stock?

[Taylor]

Where'd the cheek piece come from? Thanks.

[KnowFear]

Thanks all for the input - now that I know what to expect, I can concentrate on developing techique and consistency rather than futzing with the hardware.


Nero - How does the HiLux hold up - any problems?  I have had problems with the Bushnell 2-7x32 not holding zero on the SKS, and would like a cost effective alternative for variable magnification.  I see that they have retical options also.


Mike - the CMP supplied stock is pretty stout, so the $12 aluminum rail stock is just cut to length, drilled and counter sunk, and attached with 2 #10 sheet metal screws (cut so that they don't protrude into the interior) screwed into pilot holes drilled into the stock.  Sling mount doubles as a hand/hook stop and is the larger picatinny units at about $10.

I don't know how that would work for you - I need the sling attach point and stop close together to hold Mr. Hook in place, but it may be too tight and pinchy for a meat hand


Taylor - cheek piece is a Triple K 3/4" - http://www.cabelas.com/product/Triple-K-Leather-Cheek-Pad/706669.uts at around $30

It is a straight up rest, not the offset ones for the M1C/D.

My big stoopid neck is so inflexible that I have to build it up underneath by another 5/8", but if you have normal flexibility, the 3/4" of the Triple K itself may be fine.

[Nero]

Nero - How does the HiLux hold up - any problems?  I have had problems with the Bushnell 2-7x32 not holding zero on the SKS, and would like a cost effective alternative for variable magnification.  I see that they have retical options also.

So far, so good, but as noted I haven't really hammered it yet.  I'll post on it as soon as I've gotten 500 or so downrange using it.  When I read the reviews @ Midway, some loved it and some reported durability problems.  I decided to go with it since at that point I wasn't sure a LER would work well for me, and didn't want to pop the $ for the higher priced options.  Now that I know it will work, I might go for the Burris or Leupold in the future.