Sabot slugs: erratic accuracy as distance increases
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 (fully rifled), 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.
According to my calculations the slug should go sub sonic at around 175 yards. Could it be that as the slug slows and approach the speed of sound it begins to experience turbulence?
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.
I have run into the same problems as you. I have a Mossberg 835. Good job on the cantilevered barrel. The barrel/reciever lockup on a shotgun is iffy at best. The scope or sights on the barrel itself will end up being more consistent in my opinion. The twist rate has a lot to do with it. You might want to try a lighter slug or a faster twist barrel. I shot only two shots at a time as it seemed as everything heated up the group opened up. I no longer subscribe to the train of thought of trans-sonic turbulence. Appropriate rotational speed throught flight should keep the projectile on consistant path regadless of mach. IMO. Shooting lots of slugs is truly tough on the shoulder and the wallet! I ended up using Fedral /Barnes combo. Not impressive in the speed department, but 2" or less at 100.
AR-10T in .308 & .243 both 24"
700 LA 7 WSM 30" 5R
AR-15 Rock River lower, WOA 26" varmint upper
Savage/ Broughton 23" .308
I went through the same thing years ago when I moved to a shotgun county. I tried many combos 1100 , 870 hastings barrels and others. Then I ran into Randy Frits (spelling ?) of Tar Hunt rifles and shotguns and it became clear. He does R&D for Lightfield.
After a long talk the final result was as mentioned barrel reciever fit is iffy at best. So it doesn't matter if you shoot scope or open sights if the lock up flexes under recoil you will often get poor results or two group.
An easy fix is make it a slug gun only and have a barrel fitted. The barrel can be removed by a smith if need be.
Or have a bolt gun built but that will cost a lot more $. I started shooting an H&R Ultra while I was going to have a custom built. The lock up is tight and I never looked back. This low $ slug gun shoots with the best of them.
"Fast is fine accurate is final"
After some research, and considering other options including purchase of a new gun, I sent my 870 off to Da-Mar Gunsmithing in New York. They specialize in slug guns. They are going to pin the barrel to the receiver, recrown the muzzle, lap the barrel, remove the cantilever and replace it with a solid receiver mount, lengthen the cone, and work the trigger over to reduce it to around 3 pounds. I expect that some combination of this work should improve consistency. I can post my results when the gun is returned. Total cost is expected to be around $275.
I also live in a shotgun only area, and found that even a certain brand/type of slug will vary accuracy wise in my Tar-Hunt DSG according to lot number.
I found a lot number in SST flavor that the gun likes, so I bought quite a few boxes. The group size varied from <2" @ 100 Yds to around 6" at the same distance just by changing from lot to lot.
I read Randy Fritz's explination on how your hold will greatly affect the group size, and he's right on the money. Because of the slow mv (in relation to CF rifles) the gun starts to recoil before the slug leaves the barrel. You can prove this by shooting the gun free recoil in a Caldwell lead sled - 6" to 8" higher @ 100 Yds than if you were to hold the forend down in the same rest.
Wind has a lot of effect on the slug as well.
I switched to the DSG because after using up all my same lot # BRI slugs for my Model 37 Deerslayer, I spent more $$ on slugs trying to find another slug that the gun liked than what the gun cost, with no luck.
I know that this thread is about accuracy, but the Terminator slug by Dixie Slug is the most lethal slug that I have ever seen. Accuracy with a Hastings rifled barrel on an 870 is in the 3" range at 100 yards.
Check out the damge, it made my 416 Rgby look puny. This slug blows about a 3 1/2 to 4 inch wound channel
range it,check the wind, dial in correction, aim and only one shot