I am new to reloading by that I mean I just set up my dillon rl550b and have not reloaded anything just yet. My question is about case trimmers. Do I need one to reload handgun loads (9mm-40s&w-38special). Also what is the purpose of the case trimmer. You can call me an idiot and I will understand, Like I said I am very new to this but I find it extremely fascinating. Thanks for any help

I have not had to trim pistol cases.I load for 40s&w,38/357,41mag,44mag and the 45acp.You need one,I think if you are reloading rifle cases.

I would suggest getting some Lee Case Trimmers to keep things consistent and to spec. The Lee case trimmers are cheap and work well.

Every time you fire a case the gases push in all directions. The line of least resistance is the bullet so that is what eventually gets pushed! Up the barrel.

However before it starts its journey to the target the cases is also pushed. Outwards (side to side) and lengthwards (top and bottom).

Now the most obvious result is that after it is fired the outwards (side to side) bit has to be resized. Pushed back all around using a resizing die.

But there isn't a resizing die that can do that for the lengthwards part at the very bottom or the very top. So?

The trim die trims off from the case mouth the bit that has been pushed lengthwards. A bit like the barber trims you hair when it gets too long.

Now just like you hair it can get longer and not be much bother. And a bit longer again. But eventually it'll need a trim. So. The trim die.

Other uses are to give a nice defined cases mouth in cases that headspace on the case mouth such as 45 ACP. Indeed whenever I bought once fired surplus 45 ACP cases I'd as routine trim them all to guarantee me this defined mouth.

Also on cases like the 38 Special with a normal roll crimp the longer the cases the tighter the crimp. Now if you a batch of mixed length cases the long ones will end up with a harsher crimp than the ones of standard normal length.

So yes it is necessary for best perormance. Is it required 100%? I managed as did most for years plinking without one.

Don't buy the RCBS one. It is useless BTW! Forster is the best and as a chaepie then the Lee.

Sometimes you need to trim cases to length before they are crimped.

I shoot some really hot loads in my .41 Remington Magnum and have to crimp them pretty heavy to keep them from backing out.

If you set up the crimp on a "short" case and then crimp the rest, the longer ones may bulge.

Ask me how I know.

Oh, and I'm real happy with my Lyman. I took the crank handle off and chuck the shaft into a cordless drill. Works great. One shaft for .223 and another for everything else.

Go buy a Lymann reloading manual. In the front of the book is all the information you would ever want to know as a novice about the why's and how's of reloading. It's also written in basic terms and gives good definitions of the terms used. with lots of drawings to explain things.
Reloading is a dangerous undertaking and not being smart enough to ask questions is the most ignorant thing of all. By asking the question doesn't show you how little you know but shows how willing you are to learn and learn correct procedures. We all started somewhere.

Thanks for the info guys, I will add a case trimmer to the list of things to buy.

Should you have a case trimmer? Yes
Is a case trimmer necessary for straight wall case pistol reloading? In my opinion, no.

Like was mentioned above, the expanding gas inside the case expands the case outward in all directions. In bottleneck cases, the gas pushes the shoulder forward, thus lengthening the case every time it is fired. In straight walled cases the gas expands to the sides, but there is nothing for the expanding gas to push against to expand the case forward.

In the past 4 decades I have reloaded thousands of .38 spl, .357 mag, .44 mag, and .45 acp and I have never needed to trim any of them.

I also reload several bottleneck rifle cartridges (.22-250, .257 AI, .270 Win, .30-30, .30 Gibbs, .300 Wby, and .375 RUM) that I usually trim about every other reloading. I have two trimmers, a Lyman and a Forster.