SmallCaliberCentral

The chamber length of my 17 mach IV is 1.446”. The directions that came with the Sinclair chamber length gage indicated to maintain neck length a minimum of .024”shorter than the chamber. Therefore I could feasibly trim my cases to 1.422”.My winter project has been, reducing lake city 223 brass (been packing 750 rounds around since 1969) into 20 VT. Fun project. I am now reducing some of them to 17 Mach IV. Hornady book shows trim length 1.390- 1.400”. What are the advantages –disadvantages to longer trim length if I have the chamber to accommodate it ? Seem to me that longer would help with concentricity and help reach the lands ? Thanks, Dan

It's just my humble opinion but I prefer a longer neck. More neck/bullet contact so I can reach the lands and still have a bunch of bullet in the case neck.
What you need to do is know the "actual" chamber length. That way, you'll know when or if you need to trim.
Pick up a Sinclair "Chamber Length Gauge" insert for every caliber you shoot. ( not sure if they make one in 17 ??) Get the "actual" chamber length measurement and you'll know when you need to trim your brass. I prefer longer necks. Again, JMHO, Mike.

Dan, you're on the right track. I like long necks too, and for the same reasons you state. As you have the Sinclair Chamber Length Tool, you already know your chamber length. My drill is to keep necks as long as possible and still fit the chamber with a neck to chamber end clearence of .010" - .015". I know Sinclair advises to go with .024", and I believe that's for newbies safety sake, as .010" - .015" clearence is plenty as long as you closely monitor your case length after sizing.

So.....necking down some 223 cases to M4 eh? That will indeed require some substantial neck turning, but should give you a healthy supply of primo/stout M4 cases when you're done.

And something else to consider, what type of dies will you be using. On full length dies, the "button" will keep pulling the neck longer with each use. Consider shoulder bump, bushing dies.
Size the neck and bump the shoulder without stretching the case body. I've got BR brass with 20 to 25 resizes and havn't had to trim. The brass just dosen't grow.