Sabot slugs: erratic accuracy at distances
I live in Washington state, and several parts of the state have firearm restricted areas that do not allow centerfire rifles. I was drawn for an elk permit in such an area. I bought a Remington 870 with cantilevered slug barrel, and mounted a Nikon scope. After testing various brands of slugs (ouch on the shoulder, and the wallet!) I settled on the Winchester Platinum Tips.
What is odd and frustrating is that I can shoot very good groups, most of the time, at 50 yards. However, at longer distances such as 100 and 200 yards the groups really open up. Specifically I can shoot 1 to 2 inch groups at 50 yards, but at 100 yards they're 4 to 6 inches, and at 200 yards they often a foot or worse. They do not follow a typical "MOA" progression as my centerfile experience would suggest: 1 inch at 100 yards is 2 inches at 200 yards, and so on.
The projectile is a .50 caliber slug nestled in a sleeved "sabot". The slug weighs 400 grains. The muzzle velocity is 1700 ft/second. I've heard, but not confirmed, that the barrel twist on the 870 is 1:36. Someone told me that Benellis have a 1:28 twist, and thus will stabilize such sabot slugs better.
It could be my shooting, but I humbly suggest that is not likely as I've shot a lot of centerfires with consistent results. I changed out the scope and that does not appear to be the culprit. It could be the cantilever mount, or something with the stabilization of the projectile.