at44-10 o-ring problems

Hi guys... so this is really ^*%$#@ me off. 
I can't seam to put my barrel in place without tearing up the breach side barrel o-ring. I have used 8mm x 1.5mm replacement o-rings, I lube the rings with silicone grease and slide it in as gently as possible... only to find it still leaks and once pulled apart to inspect again, the o-ring has chunks out of it.
Where am I going wrong?