I am using 36 grains of imr4064 and it seems that there is alot more empty space in the case versus any of my other reloads is this normal.

Yes.

If you want a higher volume of powder, you'll have to switch to a slower powder.

I don't know what bullet you are using but with a 100 36 is a minimum load around 40,000 and 80% case capacity. So yep 20% empty space you can load up to 90+% with 4064 or like Slim said go to a slower powder. But if you are going to load that light I would stay with the faster powder.

a slower powder helps.....I use H-414 but either of the 4350s will do as well.

Don't be afraid to work up a a load the Roberts can tolerate a bit more than most manuals call for.

Unless you're varminting, there is every reason to switch to 4350 and 100 grain pills. The 257 Roberts really is superior to the 6m/m family.

One of my favorite loads for the 257 Roberts uses 35.0 grains of 4064 and 117 grain bullet.
They group well and I haven't had the extra space cause any negative issues.
Some say it isn't a good thing as the powder can shift in the case causing variations in burning but so far I can't detect an issue with these.
Try them at the range and see how they shoot.

Good Luck

Thanks for all the advice I will probably try and work this load with the 4064 up first and then try the h414 or the imr4350.

i use 39gr with a 100gr TSX and it chronos at 2900fps. the rifle is a m700 with a 22 inch tube. I could load more powder for more speed but I have never seemed to need it.

perry