of the (+ or -) 40 cartridges I reload for, the 5.7 X 28 out of the FN is the hardest most frustrating cartridge to reload, & has the shortest case life... it's not so bad out of a fixed breach gun like the Contender though... however, new brass is not available, ( rumor has it Starline is going to start selling new brass, weather it's pre-laquered or not ??? )... unless you are a glutton for punishment, ( or like I do, look at reloading as a completely different hobby ) I suggest selling the once fired cases, & finding your best bulk prices on loaded ammo
if you are still thinking about reloading it, here are some problems I ran into... besides finding the right powder... my early ( I think Lyman dies ) were too long, & I had to have my machinist buddy shorten them ) I had to buy 3 different shell holders before I found one that fit tight enough not to damage the tiny rim, with all the force needed to properly resize the case ( damaged rims & stuck cases were common, when I 1st started )... I had a file trim die made, as the brass stretches more than any other I reload for... I've pulled off 3-4 necks & stuck them in the sizing die, out of the 500 - 600 rounds I've reloaded...they were thinned on the inside, where the shoulder was movd from & you couldn't tell from the outside ( now I pre-inspect using a bright light & shining it into the cases, I can usually see a bright ring ) I went throuh 4-5 different lubes before I found something that worked well on the laquered cases as much as they need to be sized... some primers are staked, I just seat primers in the staked pockets, but it takes a little futzing sometimes... I had a 5-7% failure rate in once fired cases I bought, & 7-10% on twice fired, I haven't reloaded that many 3 times fired, but suspect the failure rate will be higher yet, plus it may be time to re-laquer them for proper function in the FN...
I have used some powders that there is not loading data available, using my Contender barrel as a test barrel, but as DARKGAEL indicated, the burn rate needs to be just right for the FN to function reliably... one interesting thing, is that the gun has a fast enough twist rate to stabilize much heavier bullets than what are available in factory loads, but powders & charges must be chosen carefully with various bullet weights, not only for function, but also to prevent a KB ... ( a search may turn up 4-5 guns that have KB'ed )... probably all related to reloads, so I'm very carefull, starting with the Contender barrel 1st, then working up loads I've tested in the Contender & deemed safe powder charges in .2 grain increments
