Umarex T4E HDP 50 .50 solid steel ammo

The HDP 50 is very ammo fussy and likes stuff that is good quality to stop it jamming in the internal magazine.
The Umarex rubber balls work well but it don't like the cheaper stuff, I tried some 12.7mm solid steel ball bearings that weigh in at a hefty 131 grains and they feed through the magazine perfectly, accuracy is good at 10 metres and they are travelling at an average speed of 260 fps which if you do the maths is a lot more than it should be in the U.K.