I would use a 70 or 75gr hornady or speer bullet. They are accurrate and not as expensive. You,ll want the lighter bullet to go faster and get to your marks as soon as possible. Ive shot many yellow bellied marmots,and the weight bullet will work great,you really don,t need a heavier bullet for them . vangunsmith

The easy bullet loses speed faster, at it the ballistics is worse. It blows off a lateral wind more.
It is more probability of a mistake at calculation of clicks for a scope.
The last year on tournament on marmots the strong wind was. A lay of land hilly. It was difficult to shoot precisely.

I would go with that bullet also, considering wind as a big factor. Not sure if the others like Speer or Sierra make a 100 plus grain match or varmint bullet.

I have 1:10 twist. Likely this bullet Hornady 105 grain A-Max will somersault?
100 gr flies normally.

Unfortunately, it doesn't look like there are a lot of good varmint bullets in the 100 grain range, so here are the best ones I could find. I am sure you can get them out of Cabela's.

http://www.sierrabullets.com/index.c...0&bullettype=0

The Sierra Game King has a BC of .429 over 2800 FPS.

https://www.hornady.com/shop/

The 100 grain Spire Point Boat Tail has a BC of .405

http://www.speer-bullets.com/default...1=3&s2=7&s3=21

The Speer 100 gr Boat Tail has a BC of .430

These three are probably going to be your best bet to give you long range performance on varmints, and still work in 1:10 twist.

The 95 grain Nosler Ballistic Tip would be my choice if limited to a 10" twist. Although some of the other bullets in the 95 and 100 grain weight range advertise a higher ballistic coefficient, my field experience suggests Nosler's advertised B.C. figures are pretty conservative. This bullet is good out to about 600 yards - beyond that, you really need the heavier VLD bullets. Roscoe

I have a 6mm remington (same bullets different case)and NOTHING has come close to the bergers for accuracy, I use them on coyotes and smaller. they aren't designed for hunting so I use barnes X for deer, antelope, not as accurate but great terminal performance. With marmots accuracy is all you'll need to worry about. Also, I think the heavier grain bullets will be better(90grains and up), as they will be affected by the wind, or other weather factors.

Another vote for the 85 Grain BTHP Sierra. I also like the 95 grain Ballistic tip. Midway USA is good to deal with. Have a good day.

Sasha - is there a particular maximum range you have in mind? Some of the ligher bullets will work great out to 400 yards.

Hello. The beauty of the 243, in addition to being a fine deer cartridge, is in it's superior varmit capabilities. This one will blow you away...55gr Combined Technology BT with 41.3gr IMR4895....3650'/SEC and not even close to max load. This one shoots sub MOA at 200 yards in three different rifles!!! This is so good that you just have to try it to believe it. The bonus....very little pelt damage...try it you will be happy. Regards, Rick.