Lapua D166

I'm playing with a Finnish M39 Mosin Nagant and trying to replicate the Finnish military loading with the 200 grain D166 bullet. The Lapua D166 bullet mikes .3095". Neck tension with both Prvi and Lapua brass is poor when expanded to .310 (expander provided in the RCBS die set). Using the also-supplied .308 expander helps, but gas blowback becomes a problem. You can actually feel it on your face. Cases come out sooty. The brass does not seem to want to obturate. I used Lapua brass with a worked-up-to-published maximum load of Vihtavuori N140 powder. I tried to use my Prvi brass earlier this eve, but, again, bullets can be moved with the fingers. Hornady or Sierra 174 grainers at .3105" and .311", respectively, don't present the problem. So far, I haven't tried either with the VV powder on account of data lack, but neither shoots to the sights with either IMR4064 or H380. I'd be most interested in finding out if anyone else has had similar experiences and suggestions about how to fix matters

Use a bushing die to reduce neck size and increase neck tension.