The 338-416 Rigby improved is an excellent round that will easily outperform the Lapua or ultramag case without reaching unsafe pressures. The rigby brass is also much cheaper. The Lapua is a shortenned version and will not approach the full length rigby case in performance in any improved form. It is by far the most you can get out of a hunting weight rifle.
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The Allen Xpress(AX) is a 338 Lapua that has been improved with sharper shoulder angle and less body taper. Not quite as sharp as the AI version but same performance. The idea is better feeding characteristics with same performance.
You are thinking of the 338 Ultra Maxx which was designed by the request of one of my good friends and customer who frequents LRH under the call sign of JOECOOL.
Its a 270 Allen Magnum necked up to 338 basically. Because the shoulder location is moved forward compared to a 338-300 RUM, its easiest to get 375 RUM brass and neck it down for fireforming as you can position the shoulder properly to hold the case solidly against the bolt face when fireforming.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.