Hi, I have a Mauser 98 in 308win. It has military barrel, sport stock, DIY epoxy bedding and a Timney trigger.

I used to hand load to get good accuracy but due to a severe lack of spare time I now buy ammo. At first this was an upsetting thought but I soon found that Winchester Ballistic Silvertip ammo had the accuracy I needed. With a best five shot group of 3/8" and regularly around 1/2" I was pretty happy.

But the accuracy has gone away. Now I am struggling to put all shots inside 2" or sometimes worse.

Do ammo companies change their loads? Or is it something in the rifle? Where should I start to look?

Did the change in group size correspond to a new box of ammo? If so, is it the same bullet weight and style?
If the accuracy has fallen off gradually, I would check the following:
Action screws - Front should be quite tight and the rear just snug.
Scope mounts - particularly windage screws on Redfield-type rear bases.
Barrel fouling?
Scope damage or failure - will need to mount a different, proven scope to rule this out.

The previous batch of ammo I used was over a year old. Now that I have used it all I bought the same weight and style.

Action screws are fine. Scope mounts I have not checked so I'll have a look.

Barrel fouling, I regularly clean the barrel with hoppes and with sweets when required.

Scope, it has got a fairly cheap tasco high country on it so I might change it to my leopold and see.

Any other ideas?

Yes they may have changed things. Possible design changes of the bullet , changes in the powder ( I've seen that) or primer may change accuracy in your gun.

I've never used Sweet's, but I have used Hoppe's and found Hoppe's to be about as effective sewing machine oil at cleaning out the copper fouling.
I'm betting that you've got a bunch of copper fouling in the barrel.
Jim

Don't forget to check the muzzel crown for burrs/dents.

AFAIK, hoppes is good for cleaning powder/lead residue and sweets breaks down copper. You can tell by the bright blue patches when you push them through. I will give it a clean but I have not put that many rounds through it since it's last clean.

I'm betting the ammo on the nose and each way on the scope. I think I will go through my old load data and retry something that used to work. Will my powder be ok if it has been sitting around for a couple of years? Or should I just buy a fresh batch?

The U.S. Army estimates the lifetime of properly stored powder at 50-75 years. I'm shooting 1941-42 surplus .45 ACP right now. The muzzle velocity is still at nominal spec. I'm also using surplus IMR 4895 that's as old as I am, made in 1955. Works great! The biggest enemy of smokeless powder is heat. The standard test the Army uses to check samples of stored powder is to put vials in an oven at 150�F and check them daily. Any sample that puts out reddish fumes within 30 days is at the end of its life. That's confirmed by a repeat accelerated aging test or by measuring the amount of stabilizer still available in it, and if it flunks the ammo's disposed of immediately or shot up within a limited time. The point to note is that 150�F is reached in lots of attics, storage sheds and parked cars in hot summer weather. As Hodgdon says, "If you're comfortable, your powder will be fine." Don't let it get hot, whether loaded in cartridges or not.