No but it can make it more difficult to achieve the same results. I have a couple lowers with Hi Speed NM triggers and PRS stocks on them that are much easier to shoot more accurately than a lower with a CTR or UBR stock with a very short fast HiperFire trigger. My 2 accuracy lowers are absolutely identical other than a PRS Gen2 vs Gen 3, and they shoot exactly identical.Can a new lower make a good load shoot bad? I don't see how
Not knowing his exact setup, it's hard to determine what the cause might be. All things being equal, and the lower being the only change, they should shoot identical. Any deviation from that adds additional variables.