sighting in.....Problem, opinions to fix??

cloverleaf · Oct 15, 2018

I'm currently trying to help a friend sight in a .20 ga slug gun. He is 4 inches low at 20 yds and at the top of his adjustments with no movement in point of impact. We ran out of time to shoot at 50 yards where I am hoping he will turn up "high". Otherwise which end do I shim? Also, know that this is a Mossberg 500 slug barrel with a cantilever mount. Running the new VX- Freedom 3-9. I will wait to review that scope.I don't even want to think about that cantilever being wonky. At 20 yds are we just "point blank"? - Thanks for the thoughts. CL

jimbires · Oct 16, 2018

CL , you could use the Burris signature rings I linked to in the other thread . I've used these and it worked out perfectly . if you're wanting to use a shim , you'll want to put it under the rear ring . this will point the scope down towards the bullet hole . this is what you are trying to do, get the scope reticle to align with the bullet hole . the problem I see with a shim is you are raising one ring and doing nothing to the other ring . this could cause a bind on the scope tube . the Burris rings will not bind the scope .

Nimrod84 · Oct 16, 2018

CloverLeaf,

Before shimming / changing scope rings, I would suggest making sure that the scope is tracking correctly and make sure that their isn't any divots or other issues with the barrel; I returned a Mossberg 835 barrel due to the drill used to port the barrel going too far and into the rifling on the bottom of the barrel and my father had a Zeiss HD5 that the turrets were reversed (up was down, down was up).

2ndly, I suspect that at longer range you should be high or on POA as you hope - the cantilever really raises the scope up and at 20 yards the slug should be going up. One or two rounds at 50 or more yards is the best way to check, but you could have your friend measure the middle of the scope to the middle of the barrel and see what the distance is - 4" sounds excessive to me which is why I suggested checking the barrel and scope first.

FYI - At 20 yards, each click of the scope adjustment I believe ends up being 0.02" (assuming each click is 0.25" at 100 yards) - scopes with limited adjustment can be impossible to sight-in at less than 50 yards when the scope is supper high. My math might be wrong, but a scope with 60 clicks (again each click being 0.25" at 100') might only have a 3" adjustment range at 20 yards but have 15" of adjustment at 100 yards.

3rdly, Mossberg barrels can be weird and ammo sensitive - if this is new barrel I would suggest trying a different brand / bullet style. I had an issue with a 12 gauge 835 rifled non-cantilevered barrel that I had to max out windage to get it roughly on target at 100 yards, its replacement a cantilevered 835 barrel is sensitive to ammo for elevation with a 4" spread at 50 yards and about 15" or 17" at 100 yards being common between slugs & sabots. I am finding that I tend to use the Federal TruBall slug as it is cheap, commonly available in my neck of the woods and splits the elevation difference between the worst offenders.

PS. Always double check that the barrel is tight on the receiver, a loose barrel can cause big POI changes.

cloverleaf · Oct 16, 2018

Thanks guys- you have confirmed many of my suppositions. My friend is out of town for a few days. We will try again at 50 when he returns. The scope went down 36, 1/4 inch clicks w/o a change in POI. didn't think that was a lot of travel, but probably about average. If needed, I would look to shim the ring between the cantilever and the ring. Shimming the ring seems to be asking for trouble. Thanks again. CL

"3rdly, Mossberg barrels can be weird and ammo sensitive - if this is new barrel I would suggest trying a different brand / bullet style. I had an issue with a 12 gauge 835 rifled non-cantilevered barrel that I had to max out windage to get it roughly on target at 100 yards, its replacement a cantilevered 835 barrel is sensitive to ammo for elevation with a 4" spread at 50 yards and about 15" or 17" at 100 yards being common between slugs & sabots. I am finding that I tend to use the Federal TruBall slug as it is cheap, commonly available in my neck of the woods and splits the elevation difference between the worst offenders."

Nimrod- I have had my share of woes with slug barrels myself. Im surprised that you can get Tru-balls to do anything in your rifled barrel. That's not really what they are made for. I assume you've tried Hornady FTX slugs. They have shot decent to exceptional in several Mossbergs that I've seen. Mine included. However my best performer has been federal Trophy Copper slugs. Shot a sub three inch group at 100 yards several times. My experience mirrors yours, slug barrels can be picky, and shoot slugs isn't like shooting a rifle. Thanks and best of luck with yours. CL

truck driver · Oct 17, 2018

CL one other thing to check is how tight the barrel attachment nut is. By over tightening the nut you can change the POI because it pulls the barrel down so you would run out of elevation adjustment in the scope. Been there done that with the Mossburg 500 and other shot gun slug barrels. Just finger tight is all you need.

noslerpartition · Oct 17, 2018

I think if it is 0,25@100y, it is 0.05@20y - it is linear.
Still, 36 clicks are just about 2 inches....

I have my doubts that it will be spot on at 50y. If you look at graphs showing bullet path against sight-line, it starts at the distance between middle of barrel and middle of scope, then going up, crossing the line of sight, rising over it, crossing it a second time and then going down.
4 inches low will be more than distance barrel/scope, so something seems to be out of tune with that combo

Nimrod84 · Oct 17, 2018

CL,

From what you and NoslerPartition added, I would really check the scope and make sure that the reticle is moving. I'm getting about 1.8" as what your POI should have changed at 20 yards, so if you were didn't see any change than the scope isn't tracking...

If the reticle is tracking and holding under recoil, than shimming the rear ring as you noted or switching to rings with inserts is the way to go.

My point with the Tru-Ball ammo was mainly to say your friend might want to switch to some cheap stuff while wrenching on the scope / rings or otherwise trouble shooting. They might also find a gem in the pig pen that way...

The 835 chamber is for 3.5" mags, so I think a lot of the 2.75" and 3" rounds leave too much slop in the chamber... I'm sure that is why the Tru-Balls and older style Forster & Brenneke slugs shoot about the same groups as the modern sabots for me at moderate ranges.

I have not tried the Hornady FTX sabots (co-worker had expansion issues with them and strongly recommended staying away from them); I'll add them to my list for an online order. Locally they are running about $13+ for 5 rounds and given his history with them I didn't feel like spending the money... I think I tried the Federal Coppers and they did 2-2.5" at 100 yds. I spent a good $100+ trying different sabots and they generally did about the same as the Tru-Balls and the Federal blue box in terms of accuracy out to about 60-75 yds. I think Lightfield Hybrids or Brenneke sabots shoot well for me at 100 yds (about 1.25" for 3 rounds), but I can't bring the reticle low enough to zero.

cloverleaf · Oct 17, 2018

Ya..... I hate the variables.... Thanks for the thoughts guys. I May borrow a bore sighter and see if the reticle is moving. Seems like it should have gone some where even at 20 yards. CL

PS- get that about the cheap stuff. We're using federal blue box sabots. Time for the expensive stuff later. Like you I dang near went broke getting mine on target.

Nimrod84 · Oct 18, 2018

A flashlight positioned behind the scope eyepiece with a wall 3-10 feet away usually shows if the cross-hairs are moving when the dials are turned. I've done this with the gun lying on its side on the carpeted floor of an SUV, so the gun wouldn't move around while I twisted the dials.

Typically though I will put the gun in a rifle vise and start with the illuminated cross-hairs on a light switch / door knob at 10 yards (parents house has a big basement hallway), this allows me to verify the click values at 10 yds. I also will use a flashlight shinning down the bore when bore-sighting slug barrels as a poor-mans boresighter - especially cantilevered barrels that are removed from the receiver.

Edited to add, you might want to watch the first couple minutes of - https://www.youtube.com/watch?v=lQGM8Cn8lS8 - for a visual of what I wrote. He uses the flashlight through the scope objective, which can make the cross-hairs bigger.

cloverleaf · Jan 9, 2020

Late to follow up but ran across this again and thought I would let you know.... the solution went like this: (reminder this is a 20ga. Mossberg with a cantilever mount). So I have the rifle on the bench and I am trying to take some measurements and talking with him about the best way to add ten thousandths of an inch under the rear base...and one of us grabs the cantilever and lifts up....well that's an easy .0010!! So he gets out the set of feeler gauges and lifts up on the cantilever until it springs back about that much higher...!!! Yup he just bent the thing by hand into place and adjusted the scope accordingly. I felt like an idiot standing there w/ a caliper in my hand... :roll: :lol: . A month or so later he shot a nice 8pt at 70 yards or so. CL