I have a Merkel 140AE in 450/400, built in 2011 purchased in 2021.
It has taken myself loading & a good friend shooting for me a while to sort out.
Been a lot of head scratching along the way - but fun really.
We tried AR2209, AR2213SC, AR2217 & lastly Reloader 15.
All with Woodleigh .410" 400gn softs, Federal 215 primers & Hornady Brass used in all tests.
Factory loads chronographed 1937fps so well under the 2050fps supposed regulation velocity.
My son has shot these touching at 25m, factory group at 50m was one over the other approx 45mm apart.
Powders varied the loads dramatically AR2209 strung Vertically, AR2213 Horizontally, AR2217 gave a more consistent round group at 90gn - still not good enough yet.
Tried R15, had to have a 20mm foam filler on top of 66gn, this made an even more consistent 4 shot group, velocity about 2010fps - fast enough for me!
Nothing like a H&H, but good enough to still be a Savana queen any time game pops its head up!
The real surprise was a CBE cast projectile, this LOVED 87gn AR2217!
Hard cast about F-H pencil test, fitted with gas checks ened up 384gn or so, for about the same speed as the Woodies, these touched at 50m.
Yes we've tried the LHS barrel first & it didn't double!
Lesson to learn, hook the finger well over the trigger so it doesn't slip off & tap the other under recoil - otherwise it will!
Have had a lot of fun, including a BGR competition last year, all I can say, get one if the chance presents!