Skeeter,
In my opinion, these are a last ditch effort to tame down a rifle that is much to snotty to play with.
Generally you see then used in big bore dangerous game rifles that are equip with barrel banded front sights where a brake will not be possible.
I do not care for them simply because I feel they can throw off the balance of a hunting rifle enough to make it feel unbalanced in the field.
If you are using then in a long range rifle, there really is not a great need as these rounds generally do not generate the kinds of recoil energies that demand such bandaids.
Still I will admit they do work, how well is in the shoulder of the beholder so to speak.
Basically I feel it is more the added weight to the rifle that tames recoil more then anything.
The different sizes are for different style stocks. Basically you find out the dimensions of your butt stock and see what size will fit in your stock.
I have shot a 378 Wby that has three of these embedded in the wood stock, the thing still just about took my shoulder off as the rifle still only weighed in at 9 lbs ready to shoot.
Some smiths swear by them, alot do not, I do not like them. If I am going to build a rifle in a heavy kicking round I either use a high quality brake and recoil pad or if the owner does not want a brake I install a good pad, recommend a little more rifle weight and advise teh customer to get accustom to heavy recoil.
Good Shooting!!!
Kirby Allen(50)