So I tried a bit of prone shooting last weekend using my 308Win R25, I rarely do much prone shooting.  Most of my practice is from hunting positions that I would use from a tree stand or braced up against a tree trunk seated or standing.  So not much prone shooting when hunting here in the south.  

My rifle smacks my collarbone pretty good when shooting over a bag prone, how to I prevent it.  Remington R25, 308Win, A2 stock laid out in a horse pasture.

That's tough, been there. Over time, it may give you a flinch, so it should be corrected.

You just have to experiment. Try pulling the rifle slightly tighter, moving the buttstock position by small amounts (try toward the pectoral, slightly down and inside), or using a puss pad. And not trying to bag on you, but building a little more shoulder muscle helps too.

Let us know how you fix it, and tagged for better tips.

Without being able to see you shooting in position, it will be tough to provide meaningful advice - but not impossible.

Are you using a sling? "Smacking the collar bone" as you put it, sounds as if the rifle isn't properly seated in your shoulder, or at the very least, loosely seated there. Using a sling, connected to the front sling swivel, will help to keep the rifle planted firmly in your shoulder, thus limiting the movement under recoil.

I have it tight. Use a sling also.  I have a bony collarbone and the stock rests against it. I am a skinny endurance athlete so not much meat on my chest or shoulder.  Other positions don't rap the bone like prone and only my larger centerfires do it.  My 270WSM will wake the bone right up from a prone hold.

For prone, I put the top of the buttplate underneath my collar bone where the "pocket" is.

Offhand, I put the bottom of the buttplate in the pocket.

I'm betting I know what you're doing but can't confirm without a pic. Show us how you address the rifle and we can actually help. If you're actually a skeleton without flesh then there's not going to be any help forthcoming. We don't want any skeleton snipers. It's way too creepy.

I wear a shooting coat when I shoot - it has a rubber pad sewn onto the outside of the shooting shoulder.

I also wear a very thin sweatshirt underneath of it, year round.

While they both limit felt recoil somewhat, they don't eliminate it. The purpose of the coat is one of support rather than pure padding (although it does limit it). If you're not competing internationally, then you're only limited by what you can tolerate to wear while you shoot. My initial recommendation is a crew neck sweatshirt, underneath of a lightweight canvas coat (think Carhartt, or similar).

If you're using a forehand stop, locate it as far forward as possible - it will 'drive' the rifle back into your shoulder pocket.

thanks for the tip.  I am going to try all of it out tomorrow on my day off.