Being the easiest bullet to find I'm going to mod a gun for this ammunition. My build is around rifles so what's the best anyone has found (semi-automatic)??

I did find a really nice rifle that uses .38, the gun works great but has ZERO knockdown power. It's a sniper by design, but doesn't serve that role because it takes numerous shots to even deal any damage and range isn't good.

I also have a really decent .45 sniper that serves a MUCH better role in closer quarters. I have tons of .45 ammo. And that one has knockdown power and stops most enemies in 1-3 shots.

All of my .308 snipers are super slow bolt actions, and .308 is a lot more expensive to buy and harder to find.

I have a gun I named 'Gizmo' that was a crude handgun that I've managed to mod into a cool gun with a drum clip, the best receiver you can make with gun nut II, the best scope you can make with gun nut and science, and a other modifications. It still only does 26 dam or so, but it is rapid fire (not auto) and the ammo is abundant. Of course, I'm only level 14 and I don't know anything outside my little sphere of influence in the north of the great Boston area.

They all seen quite weak, I'm considering converting my legendary Explosive Sniper Rifle to use these bullets, it would take a hit on power but it would still get just over 50 damage with additional 15 explosive damage $& knockback.

IMO, the .38 Pipe Rifles and such are excellent backup weapons. I created an assault rifle with an Armor Piercing, Automatic receiver, Marksman stock, and Muzzle Brake. Where it lacked in power, I had almost no recoil when firing it. I also had 2000+ .38 rounds. When my main weapons were low, I switched to it. I even killed a deathclaw with it and a couple of frag mines.