I purchased an OCL milkman for the HK incoming.

I need a hub mount. It’s industry standard.

If I buy a standard 3-lug from rugged I’m assuming I need both the mount and the adapter?

Looking about about 400$ it seems.

Is that correct. I cannot just buy the adapter right?

probably best over in Suppressor section. But here you go. I'm going to 'assume' the HK is an SP5 of some kind??? if so it has trilug as part of the barrel. I'm not aware of any HK 9mm that doesn't have a trilug barrel FWIW. So all you should need is a HUB 3lug. There are a zillion choices. If you like Rugged they have a new bixler style 3lug that seems quite nice (RU3). That runs about $120 it seems.
But almost everyone offers one, so it matters a bit what's important to you for features and construction and price. If you want suggestions provide some discussion of what are your main criteria.

Thank you HRC, that’s all I needed. When I searched I found the rigged titanium version. Order a Rugged universal mount. Which they say is made for HK 9mm especially.

I’m rather new to the suppressor world, got my 1st a few weeks ago. This was for one of 2 Howa rifles, 243 & 6.5.

I knew the threads on the rifle, then the threads with the suppressor. There were options at Brownells & similar suppliers. I ended up with an adapter from eBay sold by JoeBob-Outfitters, works fine.

I think I’ll put the suppressor mostly on the 16.25” 6.5 Creedmoor, it’s a little longer than I prefer on the 243. This is all for use at modest ranges, no need for 250+ yard shooting.