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I made the simple change of changing from a 139 gr flat base Interlock to the boatail Interlock also 139 gr,and my accuracy has really suffered .I am still making round holes,but shooting very poor groups..Anyone had issues like this before?.I would not think it could have that slow of a twist,but I can't figure it out.
Did you do a new load workup for the boattail bullet? Can't just swap out one component of a load and expect it shoot the same as it did before.
Depending on when it was made it could have either a 1:9.5" twist rate or a 1:11.5" twist rate. Prior to 2008 Savage 7mm-08's all had a 1:9.5" twist rate. Then in 2008 someone at Savage had a brain fart and decided to change it to 1:11.5". The 2008, 2009 and 2010 catalogs created confusion over this change as it shows all the 110-based models in 7mm-08 with the 11.5 twist and the Stevens 200 in 7mm-08 as having the 9.5 twist, but in reality they were all the slower 11.5 twist for those years (this verified by several folks at Savage).
Savage went back to the faster 1:9.5" twist rate in 2011 and that's what it's been since. That said, with the ease of swapping out barrels and the fact that Savage unloaded it's inventory of slower twist barrels to Numrich's it's best to check your barrels twist rate yourself by using the cleaning rod method to verify which you have.
I had a 6mm Remington 788 and I did what you did and had the same problem but worse. The boat tail Sierras were shooting way right and half of then were off the paper.
I got home and the powder marks were not symmetrical on the muzzle. I got a magnifying glass out and there was a bur in one lands and it was rough.
And the gas flow around the boat tail bullet when it exited the muzzle was not even and the unequal pressure was tipping the bullet.
Meaning a boat tail bullet has more surface area on its base and is more sensitive to gas pressure pushing on it base. Lapua makes a rebated boat tail that is less effected by this.
The same thing happens on old milsurp rifles with worn muzzles and flat base bullets give the best accuracy.
Bottom line, check your muzzle with a magnifying glass "before" you clean the muzzle and look at the marks on the barrel.
Below is what I mean in the photo below, the marks on my new rifle were not symmetrical. And one was wider and longer than the rest and this lead me to the bur on the crown.
http://4.bp.blogspot.com/_ICuLB148bJ.../IMG_8077a.jpg
You can also check the crown with a Q-Tip and if it grabs the cotton tip you may have a problem.
http://www.larrywillis.com/rifle-barrel.jpg
After you make some checks of the crown and find nothing then worry about the twist rate.
I have tried two different powders IMR 4064,and IMR 4166,at a few different charge weights,and have messed with the seating depths as well,and can't even get a semi respectable group.I have had it for a few years,and it never has been my best shooter,but it has always been plenty good enough for a sporting rifle for the range I hunt with it.I do need to check thence twist when I get the chance,and if it is a slow twist I will order some 130 gr Hot-Cor flat base.I can't imagine why they would ever go that slow in a 7mm08.I should have mentioned that this is an Axis that I bought used a few years ago.
I have had great success with 41 grains of R15
I checked my twist,and it is 1-9.5,and I can't see anything wrong with the crown.I have read that some rifles won't shoot boatails good even with proper twist, If this is true then I have one.I placed an order for several boxes of different caliber bullets with Mid-South and the 130s that I was going to order were on back order,so I bought a couple boxes of 145 gr Speer HotCor,and I am glad I did,it groups them about an inch at 100 yards, which is plenty good enough for my needs for this rifle.I expect I could do better with more experiments with loads,but I am satisfied enough .Thanks for the good advise folks.