jkmccoy Posted October 5, 2013 Share Posted October 5, 2013 I'm headed for the range tomorrow for some more testing, but this is bothering me so much that I thought I'd ask your opinions (sorry for the long story)... Will (my son) and I recently completed building a new AR for 3-gun competition (open). We finished an 80% lower, Jard trigger, DPMS slickside upper, AR Stoner 18" stainless barrel, we built the comp, Samson 15" Evolution rail. A scope on top and a red dot (Optima) offset on the rail. When we first put it together I put an old Weaver K4 scope on top in a pair of Nikon cantilever rings. Never intended that to be the final set up, but we could use it. The accuracy wasn't as great as we'd hoped, but it was acceptable. We were shooting 1.5-2" groups at 100 yards from a bench using only a front rest. Also, although the groups were bigger than I'd hoped the groups were reliably round. We both shot the rifle in a match at East Alabama Gun Club last weekend and the rifle performed just fine. This week I acquired a Burris PEPR mount (quick detach) and a Vortex Diamondback 3-9X. (Say whatever you want abou the choice of scopes, at least it's much better glass than the ancient K4). Yesterday evening, I mounted the PEPR and the Vortex and figured that the better glass and higher magnification would at least let me do better from the bench. While I had the scope off of the rifle I also removed the Samson rail and reinstalled it. When I first installed the rail it wanted to point up when I tightened it down. Yesterday I got it lined up right and tightened down so it's parallel with the barrel. I went to the range today. I was shooting at 50yds. from a table in one of the pistol bays because there was a .22 benchrest match on our rifle range. They were not at all interested in sharing the range with someone shooting an AR with a noisy comp. I was using front and rear sandbags and had a pretty good rest. I started shooting reloads with 55 BTFMJ's. A load that we've been shooting in this rifle. Immediately I had problems getting on the paper. I finally moved to a 15 yard target and realized my shots were very low. I adjusted the scope and moved back to 50 yards. On the paper, but still low. I adjusted the scope some more - and ran out of adjustment while still 9" low at 50 yards. I took the scope mount off the rifle to see if maybe I hadn't gotten it installed straight and found obvious wear marks on the top of the receiver and the bottom of the mount. So, I tightened the screws and re-installed although I couldn't feel any movement in the mount. It's still shooting 9" low but I have some loads I want to test with 69grn bullets so I set up the chrono and start shooting. I hadn't really shot a group yet, Just a couple of shots here and there trying to get roughly zeroed. I started shooting 5-shot groups with different loads. The groups were terrible! Lots of vertical stringing. Five to six inch groups at 50 yards. Several groups with two shots in one hole, two shots in another hole, one shot somewhere else. Of course, I'm working through a pressure series with a bullet I've never shot so I don't really know what to expect. Part way through I remove the scope mount, tighten it again, and re-install (I still couldn't feel any movement in the mount or scope). I finish the pressure series and didn't get a decent group with any load. Then I decide I should test the 55 grain ammo that I've been using. I get the same results. Five to six inch groups with lots of vertical stringing. I tried shooting a couple of groups with the red dot and the groups were better, but not great. I've removed the scope and mount and remounted the Vortex with the Nikon cantilever rings. I'll go back to the range tomorrow and see if that helps. Questions... Could the PEPR mount be the problem? Because I couldn't get sighted in (all shots way low) it looks like the mount doesn't fit this rifle/scope combo. I know it was moving some because there were wear marks. I tightened it and couldn't feel any movement but I don't know if it was ever really solid. Could the rail be causing a problem? It's the only other thing I changed yesterday. To get it to set straight with the barrel I had to hold it quite firmly back against the front of the upper as I tightened the screws. If it isn't one of those things, what would cause a sudden loss of accuracy? Thanks, Kelly Link to comment Share on other sites More sharing options...
GreenDragon64 Posted October 6, 2013 Share Posted October 6, 2013 It might just be me, but it sounds like you've mounted the scope on the handguard rail instead of the upper? Link to comment Share on other sites More sharing options...
jkmccoy Posted October 6, 2013 Author Share Posted October 6, 2013 This isn't my first rodeo. The scope was mounted on the upper (not on the rail). I don't know what part of my post would have suggested otherwise. Kelly Link to comment Share on other sites More sharing options...
sschultz Posted October 6, 2013 Share Posted October 6, 2013 Try using the other scope with the pepr mount and see if you still have issues. And try taking the scope off and putting back on again. Schultz Link to comment Share on other sites More sharing options...
iainmcphersn Posted October 7, 2013 Share Posted October 7, 2013 I had a friend with a similar problem. Turned out for him that the front screw for the mount was not in one of the picatinny slots, but had been positioned at the front of the upper where one groove is skipped. This pushed the mount (and the front of the scope) up causing shots to go low. Also since the mount could not be tightened properly, groups were horrible. Link to comment Share on other sites More sharing options...
ihocky2 Posted October 7, 2013 Share Posted October 7, 2013 I have a PEPR mount, non-QD and had declining accuracy with it. Turned out that all of screws had backed out enough to allow it to move. I think with the PEPR you have to really crank it tight. It moved my POI by about 6 inces. Link to comment Share on other sites More sharing options...
cajunautoxer Posted October 7, 2013 Share Posted October 7, 2013 I check the screws when packing my stuff for a match. Most of them backed out on me once Sent from my iPhone using Tapatalk Link to comment Share on other sites More sharing options...
bpowe Posted October 7, 2013 Share Posted October 7, 2013 Kelly, How did your range session with the Nikon rings turn out? I'd really like to give Pepr mount advice... but I've never even SEEN one. I've always used Larue mounts, having been an RO at the Larue Multigun matches I have several of their mounts. Bill Link to comment Share on other sites More sharing options...
jkmccoy Posted October 7, 2013 Author Share Posted October 7, 2013 (edited) Thanks for the suggestions and sorry I didn't report back sooner. I've been out of town on business. I mounted the Vortex scope in the Nikon rings and we went to the range on Sunday. Same results. Terrible accuracy and lots of vertical stringing. The targets look like what I once saw with a bolt gun where the wooden forend had warped and was pressing against the barrel unevenly. There would be a couple of shots close together, a couple of shots close together but inches away, and sometimes another shot somewhere else. We were shooting Sunday from a solid bench with front and rear sandbags and Will and I both got the same results. We tried 50yds. and 100yds. We tried different magnifications. I loosened the rail and slid it forward about 0.020" so it wouldn't be touching the front of the receiver. No change. I did forget to take the old scope to the range, so I couldn't make the definitive test of remounting the Weaver scope and shooting that combination. I won't be able to get back to the range before Saturday, so I won't have those results for a couple of days. Because I can't figure out what would have caused the accuracy to drop off so dramatically and so suddenly I'm thinking something's wrong with the new scope. Bill, if you have extra Larue mounts you could send one this way. I appreciate the suggestions about possible problems with the PEPR. Because the problems didn't go away when I changed mounts I'm leaning away from the PEPR as the main issue. Kelly Edited October 7, 2013 by jkmccoy Link to comment Share on other sites More sharing options...
ericm Posted October 7, 2013 Share Posted October 7, 2013 (edited) see below ericm Edited October 7, 2013 by ericm Link to comment Share on other sites More sharing options...
ericm Posted October 7, 2013 Share Posted October 7, 2013 This isn't my first rodeo. The scope was mounted on the upper (not on the rail). I don't know what part of my post would have suggested otherwise. Kelly Thanks for the suggestions and sorry I didn't report back sooner. I've been out of town on business. I mounted the Vortex scope in the Nikon rings and we went to the range on Sunday. Same results. Terrible accuracy and lots of vertical stringing. The targets look like what I once saw with a bolt gun where the wooden forend had warped and was pressing against the barrel unevenly. There would be a couple of shots close together, a couple of shots close together but inches away, and sometimes another shot somewhere else. We were shooting Sunday from a solid bench with front and rear sandbags and Will and I both got the same results. We tried 50yds. and 100yds. We tried different magnifications. I loosened the rail and slid it forward about 0.020" so it wouldn't be touching the front of the receiver. No change. I did forget to take the old scope to the range, so I couldn't make the definitive test of remounting the Weaver scope and shooting that combination. I won't be able to get back to the range before Saturday, so I won't have those results for a couple of days. Because I can't figure out what would have caused the accuracy to drop off so dramatically and so suddenly I'm thinking something's wrong with the new scope. Bill, if you have extra Larue mounts you could send one this way. I appreciate the suggestions about possible problems with the PEPR. Because the problems didn't go away when I changed mounts I'm leaning away from the PEPR as the main issue. Kelly So much for copius amounts of rodeos ericm Link to comment Share on other sites More sharing options...
Blaise1 Posted October 7, 2013 Share Posted October 7, 2013 Have you checked to see if your gas block is hitting your forearm? Are you holding the rifle with the same pressure every shot? Link to comment Share on other sites More sharing options...
Stlhead Posted October 7, 2013 Share Posted October 7, 2013 Is your cinch too tight? I have often found that an overly tight cinch will have a negative impact on all sorts of things related to rodeo. Link to comment Share on other sites More sharing options...
Flatland Shooter Posted October 8, 2013 Share Posted October 8, 2013 (edited) I've got a Vortex scope on order. I'm hoping in your case it turns out to be anything other than a bad scope. But if it is the scope, they seem to have an excellent warranty. Hopefully they back it up with excellent service. Any other scopes around the house (in addition to the old Weaver K-4) that you could try? Especially with a different set of rings. Bill Edited October 8, 2013 by Flatland Shooter Link to comment Share on other sites More sharing options...
rangerguy Posted October 8, 2013 Share Posted October 8, 2013 I think we need some pics for proper advice. Link to comment Share on other sites More sharing options...
kellymc Posted October 8, 2013 Share Posted October 8, 2013 Maybe a pic of the Samson rail. Link to comment Share on other sites More sharing options...
ac4wordplay Posted October 8, 2013 Share Posted October 8, 2013 ... While I had the scope off of the rifle I also removed the Samson rail and reinstalled it. ... Cause of problem? Link to comment Share on other sites More sharing options...
gerritm Posted October 8, 2013 Share Posted October 8, 2013 How many rounds do you have on this rifle and the comp? I had some severe carbon build up in one of my compx closing off some of the holes and deflecting the bullet causing very poor accuracy. Actually caused some of the rounds to keyhole at less than 50 yards. Cleaned the comp and all was well. Just a thought happened out of the blue. gerritm Link to comment Share on other sites More sharing options...
jkmccoy Posted October 8, 2013 Author Share Posted October 8, 2013 Again, I appreciate all the suggestions. The rifle only has about 500-600 rounds through it. I'm pretty sure that something changed that caused a big change (for the worse) in accuracy. I'm sure that the gas block isn't hitting the rail. I've checked the comp and the end of the barrel and there isn't much carbon build up. I did think that removing and re-instaling the rail might have been the problem, but Sunday when I moved the rail away from the front of the receiver it didn't change anything. The PEPR mount apparently wasn't the problem because the problem didn't go away when I changed to a different scope mount. I don't think I can do anything else until I get back to the range with a different scope and that won't happen until this weekend. Cheers, Kelly Link to comment Share on other sites More sharing options...
CJW Posted October 8, 2013 Share Posted October 8, 2013 Sorry if this is irrelevant as I am just quickly skimming the thread, but I had vertical stringing problems (started small and got progressively worse) that turned out to be a cracked barrel nut. Link to comment Share on other sites More sharing options...
PacMan Posted October 13, 2013 Share Posted October 13, 2013 When we first put it together I put an old Weaver K4 scope on top in a pair of Nikon cantilever rings. Never intended that to be the final set up, but we could use it. The accuracy wasn't as great as we'd hoped, but it was acceptable. We were shooting 1.5-2" groups at 100 yards from a bench using only a front rest. Also, although the groups were bigger than I'd hoped the groups were reliably round. We both shot the rifle in a match at East Alabama Gun Club last weekend and the rifle performed just fine. Old scope + Nikon rings = no issue. Thanks for the suggestions and sorry I didn't report back sooner. I've been out of town on business. I mounted the Vortex scope in the Nikon rings and we went to the range on Sunday. Same results. Terrible accuracy and lots of vertical stringing. The targets look like what I once saw with a bolt gun where the wooden forend had warped and was pressing against the barrel unevenly. There would be a couple of shots close together, a couple of shots close together but inches away, and sometimes another shot somewhere else. I appreciate the suggestions about possible problems with the PEPR. Because the problems didn't go away when I changed mounts I'm leaning away from the PEPR as the main issue. New scope + Nikon rings = Same problem as New scope + PEPR Therefore: 1) PEPR mount IS NOT the problem. 2) New scope may be the problem. 3) Or your rifle is the problem. Most obvious thing to check the 'home built comp' - Take it off and shoot with a naked muzzle to see if the problem goes away. Link to comment Share on other sites More sharing options...
jkmccoy Posted October 14, 2013 Author Share Posted October 14, 2013 (edited) I think I'll try golf as a hobby. It seems to be almost as addictive and nearly as frustrating as 3-gun competition. Will and I went to the range today. We mostly shot at 50yds from a solid bench with front and rear sand bags. We used the Vortex scope in the Nikon rings. We removed the scope and mounted the old K4 in the Nikon rings. We cleaned the bore thoroughly. We removed the handguard/rail. We used different ammo. We fired about 20 five-shot groups with similar results. Poor groups with lots of vertical stringing. I've attached some pictures below. The two groups in the pictures were shot at 50yds. I'm not sure it was clear in my original question (because I asked about possible problems with the PEPR mount), but the rifle was shooting pretty well and then suddenly it wasn't shooting well. Shooting it without the comp is a reasonable suggestion, but it was shooting fine with the home-made comp. A cracked barrel nut sounds like a real possibility, but I can't find a crack. What would cause a sudden loss of accuracy in an AR? Edited October 14, 2013 by jkmccoy Link to comment Share on other sites More sharing options...
mpeltier Posted October 14, 2013 Share Posted October 14, 2013 (edited) Loose bbl/ bbl extension or problem with crown/comp provided the bbl just isn't shot out. Edited to add that realizing you only have 500-600 rounds thru the bbl its not likely shot out. Edited October 14, 2013 by mpeltier Link to comment Share on other sites More sharing options...
Flatland Shooter Posted October 14, 2013 Share Posted October 14, 2013 Was the scopes switched out between the two pictures? Are those pics representative of all 20 groups shot? You mentioned you used different ammo. Is some of it factory? Something is very wrong and there is some movement somewhere in the system. One thing you have not tried is shooting the two scopes on a different gun and see if a problem develops. It doesn't need to be an AR but I would recommend a gun that has some recoil to it. I've had shot out barrels shoot better than that at 50 yards. Bill Link to comment Share on other sites More sharing options...
MarkCO Posted October 14, 2013 Share Posted October 14, 2013 Loose bbl/ bbl extension or problem with crown/comp If it is not an optic issue, Mark has it nailed. Link to comment Share on other sites More sharing options...
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