MPI shifts.....I have a problem!!

[Brown Dog]

Grateful for any ideas/ advice as to what's causing the MPI shifts I observed today:

I zero at one distance.
Move to a new distance; dial my anticipated come-up; and hit high & right.
Happened repetitively today. Happened to me on 2 previous occasions. Never figured out what it was that time either!

The resulting comeups are a nonsense; 308 165BT @ 2720fps

I would expect (0.1mil clicks):

100yds.........200yds..........300yds
0.................4.9clicks......13.9clicks

What today gave was:

100yds.........200yds........300yds
0.................2 clicks......8 clicks


opening group at 100yds:

zero at 100yds:

move to 200yds, dial anticipated comeup, hit high right:

zero at 200yds:

move to 300yds, dial anticipated comeup, hit high right:

zero at 300yds (well, one click right to be applied!)

Groups were fine, Avg 0.6 MOA at 100yds
Avg 0.4MOA at 200yds
Avg 0.3MOA at 300yds

Whats going on??! I don't trust these new comeups!

[All groups fired on similar surfaces, Bipod on grass, rear sticks on cloth/gravel. Scope NOT canted!

Should also point out that since the rifle was 'A1' it has been threaded for a suppressor (with a few fine scratches being left on the lands /ubbthreads/images/graemlins/mad.gif) and it has also been reglued to its chassis.....despite this the group sizes seem OK, just not where I want them!! ]

 

[ds]

I`d start easy. Check and reset rings,mount and base (if applic). Putting it another way give them a teewk with the allen key and then re test. If the problem goes away great if not you have eliminated a couple of the suspects.

David.

 

[Pete Lincoln]

Was the modeartor tight? is it threaded square to the axis of the bore? if you didn't vave the mod on, was the thread cap tight?
Are you still using them crappy AI mounts?
are the mounts tight, are they slipping on either the scope or the base? if your scope isn't glued in with acraglass or something like that i ca gaurantee you will get some time of other sooner or later slipping scope in rings if you are still using that AI mount.
Finaly, clamp the rifle and check the scope is tracking, i doubt that the S&B PMII is causing problems but your adjustment BDC clamping screws might be letting the BDG slip, i had that when i didnt to tighten both screws properly during zeroing and then forgot there after , every time i wound back to zero, i was nudging the BDC past the 0 and gradualy ending up with elevated POI.
cheers Pete

 

[Brown Dog]

Thanks Pete and ds,

Just been out; the error occurred again but in the other direction:

I reckon somethings wrong with the scope! ...thoughts?

 

[Scott Sharman]

BD

Dont know if this will help or not but I saw some similarities b/t your pics and this post on GGVG.

http://www.varminthunters.com/forums/ggvg/posts/554968.html

Scott

 

[Buffalobob]

If you are close to the top or bottom of the max adjustment or close to the far left or right you could be getting into a couple of problems. If that is the case, take the scope off and shim it. I re-shimmed a scope last night in about an hour and a half. I will have to shoot it and then may need to reshim again, but when you need all of your elevation it is sometimes a little trial and error involved.

One other thing being as I did not fully understand what you were trying to say. If the barrel is getting warm and walking on you at the same time as you are adjusting then you could get some confusing results. If you know the barrel does not walk then OK ignore that possibility.

 

[Brown Dog]

Scott and Buffalobob,

Many thanks for your ideas. The rifle/mounts etc are all as they've always been; less muzzle-threading of the barrel. I was shooting it to see if the -not too well done- threading was having any detrimental effect. at 1,2 and 300 yards everything seemed fine. All four groups at 300yds were each 0.3MOA; so I reckon the rifle's fine. It's just that each time I made an elevation increase on the scope, I got a lateral displacement of MPI. On the first target set it was to the right, the next day it was to the left; I also noticed at 300 that a '2 clicks left' correction gave me about double the correction it should have done; so i reckon sonethings come adrift inside the scope.....Packaged it up and sent it back to S&B (who, by the way, seemed extremely helpful) yesterday.

I go out to test the rifle, it groups fine but the scope packs up....
Is there a word for 'negative-serendipity'?!