I also plan on reloading for my .308 Gap-10 with Noveske Swithcblock. I have ordered/received 400 Lapua .308 brass, 1000 Hornady 178 Grain AMAX and plan on using the plethora of Large Rifle Magnum primers that I have. I also have several pounds of RL15. My Thunderbeast 30BA suppressor will be here in the next 2-3 months but it will be November-December before my Gap-10 is here. I was planning on loading them to Mag Length when getting started but would appreciate any help in advance.
You should have no issues running handloads through the GAP. When I first got mine it didn't like certain factory ammo, but it always fired my handloads. Now that it's broken-in it'll eat pretty much anything I feed it.
I was running a Dillon 308 die and it sized fine. But it didn't provide the neck tension I wanted. I absolutely COULD NOT polish the expander ball (carbide), so I decided to switch dies. I went with a Lee full length sizing die, which uses a mandrel instead of a ball. I was able to polish off 0.002 which provides me a little more neck tension. The reason I did this is because I don't crimp and my initial loads were getting some set-back upon chambering.
I've handloaded 168 and 178 AMAX, 175 SMK, and 168 Barnes TTSX. Powders I've used are AR Comp, Varget, 4064, 8208, and TAC. Primers I've used are CCI 34 and CCI 200. Brass I've used is Lapua, Black Hills, LC, and Nosler. All of these components have worked perfectly.
The gun will consistently shoot sub-MOA with little effort on my part. The best group I've achieved to date was 0.172" (measured outside-to-ouside and deducted .308). This was a 5 shot group fired at 100 yards. I could cover the whole group with a penny. The load was 168 AMAX (seated to 2.810 OAL), 44.0g Varget, CCI 200, and 2X fired freshly annealed Lapua brass. I shot a similar 5 shot group with 175 SMKs but got one flier that boogered the group. That load was 175 SMK (same OAL), 43.0g Varget, same primer, same brass. Both loads produced the most consistent MVs out of others I've tested. Take my experience for what it's worth,
but ensure you work up to those loads if you want to try them.
My GAP-10 is chambered in 308 WIN, has an 18" barrel, no muzzle device, and a Noveske Switch Block.