I just bought a slug barrel for my remington 870 12 g, and with 3in slugs, it kicks bad. all i need to reduce recoil for is sighting in, once i starthunting, i wont feel it as much.if i put a 25lb bag of lead shot between my shoulder and the stock, would it slow the kick down, or only make it worse?

Suggestions:
1). Slip on recoil pad (like from Sims for example)
2). Recoil pad for your shoulder (like a PAST pad)
3). Caldwell lead sled for range work
4) Why use 3" ? I've killed tons of deer with 2 3/4"

Get the Shooters Friend Recoil pad that Cabelas sells. Its about 18.00. Its a slip on pad that they clain reduces felt recoil by 70 percent. I have two of them and its BY FAR the best product I have ever seen. I have Limb Saver and other recoil pads but they do NOTHING compared to the Shooters Friend pad. Go to Cabelas site and type in Shooters Friend recoil pad and look at it. You will see right away why it works BUT only feeling the difference will make a believer out of someone.

I have a shirt from PAST that has a pad on the shoulder. Past also makes slip on pads that help with felt recoil.
You could reduce the recoil from the shotgun by adding weight but that makes carrying the gun across fields more difficult

Try and get your hands on a shoulder pad and/or recoil pad. Until then, you can loosely fold up a hand towel.

A proper fitting stock will help also

If you have acess to a life preserver/jackyou can put it on and it will help alot. I saw a guywearing one at the range one time and started doing it when I was sighting my mossberg 835 shoulder tenderizer as I called it.1-2" of foam padding takes alot of impact.I actually traded that mbin for an 870 w/thumbhole stock and you wouldn't belive how much that thumbhole stock helps...if your interested in buying a new butt stock.

I like aPAST shoulder pad best and I think it works the best for your shoulder. I'm 5'3", 145 lbs and can shoot 12ga. slugs all day (if I want to) with a shoulder pads on.

I agree, get the Past shoulder recoil pad (a strap-on device). I shot 40 rounds at the range two weekends ago and developed a notable yellow bruise on my shooting shoulder. I shot 40 rounds at the range yesterday and it was no problem, much more comfortable.

I'll join in and recomend the strap on pad. Without it I would not shoot anything more powerful than a 30-30 off some benches.
What you should look at also is the height ofthe shooting benchesyour using and see if they are hammering you atextreme angles. The tableshould be chest high when sitting so that you don't have to crouch so far when your shooting. Whoever decided that shooting benches should be the same height as the tables you use for writing had a real sadistic steak in him. My one clubrange uses proper height tables but the commercial indoor range I use and pay by the hourhas benches that are a little bit above my knees when I sit, a good height for taking a written test but certainlynot shooting.They put a world of hurt on you when you shoot without a pad to the point that I would not use them period without a pad. On them a 30-06 is brutal while the same gun on the other higher bench is easily okay for a box of ammoeven without a pad.take care and hope it helps.