IIRC .223 is actually 5.7 diameter so could you use .223 bullets to reload 5.7x28?

.223 projectiles are too heavy for the 5.7 handgun.  You risk a very dangerous over pressure situation.

By all accounts the 5.7 is about the most difficult cartridge to reload properly and safely, because of it's unique pressure profile.

It's not like loading a .45 ACP at all.

From what I've seen here, and at the FN Forum, every kaboom of the 5.7 pistol is directly traceable to hand loaded ammo.

Hand loading also voids the warranty.

Do what you want, but with this handgun the safest bet by far is sticking with factory loaded ammunition.

I am a reloading instructor with 35+ years experience.

The 5.7x28mm is a semi-auto version of the 22 Hornet - and use the same diameter bullets as the 223, 5.56mm, 22-250 etc etc etc etc etc - it is a .224" bullet.  You just cannot use the same bulelts from these other calibers in a 5.7 'cause the 5.7 likes 27gr to 40gr bulelts - 55gr for subsonic.

I have a ton of experience loading 223 for NRA Highpower Rifle competition to 600 yards - and I can tell you the 5.7x28mm is a very very difficult round to reload.  The factory outside coating interfers with full length sizing.  The case is too short to accomplish FL sizing, de-priming, and neck sizing all at once like normal  reloading easily - your better off performing these steps individually, or, wreck 20+ cases getting the die dead on perfect.

The full sized brass always needs trimming.  The primer pockets are military crimped and need to have it removed with a swagger.

The big problem - FN has never relieased their pressure data to United States manufacturer's - this means none of the powder or bullet companies can publish a load with 100% confidence it is safe.  I've talked at lenght with Hodgon and Sierra about this.

There is some load data on the internet, and, a few magazines - but these almost always use a local powder like 'True Blue' that is only found on the East Coast and not on the West Coast, Alaska - or sold by mainstream companies like Midway.  This makes loading for this caliber very dangerous!!

Bottom line.  There just isn't enough info available for anyone to reload this round 100% safely.  Just take a peek at your factory once fired brass and you will notice they all show signs of over pressure.   I've never blown up a rifle or pistol reloading in 35 years, but, this is a real possibility for me as I continue to work on this caliber.  By the end of June I should have some safe loads for 30gr Midway Dogtowns, 40gr Nosler BT's, and 55gr subsonics for people using suppressors.  I will post them in this Five-Seven area.  Lots of nice folks here.  I have 24/7 access to a nice range just minutes from my house..

American gun shop walls and ceilings all over the country are decorated with blown up rifles and pistols - mostly from reloading.  These are guns blown up that had published data available.  The 5.7x28 is not for the novice reloader - and - if you can't afford to have your gun blown up - you shouldn't be reloading for this either.  

Hope this helps!  You can email me at the end of June and see where my load data is at.

call it a 22 K Hornet and I'll agree with you   ;)  Anyway, as for reloading, yes, this round presents some unique issues due to the design of the firearms.  For a large reloading discussion for the 5.7x28, go here: http://www.fivesevenforum.com/viewforum.php?f=19&sid=d6053aec7e0bfdde6c66a5110e1dc0f2

I have had no problems with the resizing though.  I clean then lube the cases then size and deprime them.  They do need to be trimmed more often than the .223, but I dont see that as a problem.  I never swage the primer pockets as it is pretty easy to just seat in the new primer.  I purchased a powder measurer that is well known for its accuracy as this round can be very sensitive to very small changes.  As for bullets, I have loaded 30 and 36 grain Varment Grenades, 35, 40 and 45 grain balistic tipped bullets and I intend to load 50 and 55 grain ballistic tipped as well.  As for the flat base bullet thing, not true.  You can load boat tails in it just fine.

The large shoulder movement you seee in this round is a product of the design of the firearms.  Even low powered factory ammo has a very large shoulder movement in comparrison.  Because of this, the number of safe reloads you can do with this round is between 3 and 9 depending on how stiff the loads are.