i am new to airguns but have just bought a beeman 850. can anyone tell me the difference in the 850 and the 900?

Beeman 700 = Diana 5, 800 = Diana 6G, 850 = Diana 6M, 900 = Diana 10.

The 6M and 10 have the same powerplant and barrel group. But the model 10 is a much more advanced gun with completely re-designed grip frame, lower center of gravity, and multi-adjustable true match-level trigger, grips, and rear sight.

The 6M is a good entry-level target pistol. The model 10 has performance similar to famous Feinwerkbau 65 and was one of the world's most advanced air pistols in its day.