I know there is a strong opinion that slower powders always give faster velocity. But I'm still trying to understand why it's true for the short barrel. (That is the trouble of all my life – every time I try to understand. People say – just do it; my answer usually is "No way before I understand what I am doing").
Well, it is not
my opinion that slower powders produce greater velocity even in shorter barrels, it is a matter of fact and located in printed material that is over 3 decades, no 4 decades old. If I was espousing
my opinion on a subject, I would have put that as part of the comment. Get a loading manual and check it out. Call Hodgdon or Alliant or Accurate or one of the other powder companies and ask their technicians. You will get more velocity, period, with slower powders. You will also get more muzzle blast or flash, period.
Not opinion, fact. And believe me when I say this, I want people to KNOW what the believe and WHY they believe it. Not just about loading ammo either!
Just so you do know though, it is because the slower powder's gas expulsion is still expanding even in the shorter barrel. The faster powder is usually done by the time a bullet leaves the case. (Slight exaggeration, but you get the point.)
In case of the fast powder all powder will be burnt while the bullet is still inside the barrel. In that case most energy will be taken by the bullet. From the other side the slower acceleration is easier than the fast acceleration. So in my opinion the ideal powder should burn exactly until the moment when the bullet leaves the barrel. The faster is bad (fast acceleration attempt and unreasonably high pressures). And the slower is bad because some part of the powder load will not participate in the acceleration process but will just produce the spectacular firework (which is nice but not my actual goal). That is why (in my opinion) the best powder for 3" barrel should be faster than the best powder for 10".
I'm a newbie in reloading and could be wrong. Please explain me where?
I need the powder and bullets recommendation for two opposite 500 S&W loads for exactly 4" barrel:
1. Target load with manageable recoil and inexpensive (but FMJ or plated) bullets;
2. Max killing power (500-600 gr.) for bear-defense (black and grizzly); here I don't carry about recoil, I just want to stay alive!
I got the part about being a newbie. We all have been from time to time. Keeping a humble attitude is the key to learning from "grizzled old warriors" of the hobby though.
As for your loads:
From one extreme to the other can be done with the 500. I used Trail Boss with a 350gr Speer Gold Dot for 1200fps from a Handi-Rifle. Probably give you 900fps from a 10" handgun and less than that from your 4".
I also used the same bullet over a full charge of Lil' Gun in the same rifle for just over 2100fps. Now, I don't know what kind of bear live in your area, but if a 350gr bullet traveling 2100fps don't git it did, youse in the wong place!
(Get a reloading manual for the data you need.)