Testing Rounds in the FN303

 I recently finished rebuilding my arsenal of rotten seal FN303s, they weren’t all broke but it seemed easer to just have all new o-rings all around.  After rebuilding I gave them a few dry fires in my office just to test function.  When I finally got a chance to shoot them the first three all failed to function, strangest damn thing.  Turns out that I left the regulator screw at the rear a little too loose on them all.  I haven’t sorted out just where the sweet spot is but you want to tighten the regulator screw until it has slightly compressed the spring pack inside and then get the check nut nice and tight to keep it in place.  This will give excellent velocity which will put a nice dent in a hard wood post at 30 yards.

Seeing how the FN made projectiles are very difficult to find and ridiculously expensive at $2.85 a round for paint and $4.65 a round for PAVA it seemed important to find some other options.  A little bit of searching showed that the FN303 was unsuitable for “normal” paintball rounds but I couldn’t find a lot of solid info, there don’t seem to be a lot of folks out there using the 303 to a significant degree.  In my search for info I started an account at PbNation.com “Mag Fed and Mil-Sim” forum where I received several suggestions on alternative projectiles.  The first was a brand of specialty paintball called First Strike (FS) which bears a strong resemblance to the FN rounds.  The second was essentially a solid plastic version of the FS rounds called RAP4 that are sold as a low cost alternative for training.  The third was a solid rubber ball which is apparently either used as a reusable training projectile or for crowd.

To start with I can tell you that a ~3/4″ long peices of 5/8″ wooden dowel can load up in the mag and will fire just fine. Velocity seems a little lower than FN rounds as its slightly undersized for the bore but the real problem is with accuracy. For the first 15-20 feet the wooden slug will fly somewhat true before tumbling and veering off in random directions. At thirty feet I don’t think it would be possible to hit a three foot diameter target.

The FS rounds were something I had originally found in my google searching and was subsequently recommended by the PbNation forum.  They are available from a variety of paintball sites however I bought them off Amazon as the prices (34 cents per round) seemed the same everywhere else and they had free Prime shipping.  Superficially the FS rounds look nearly identical to the FN rounds however structurally they are significantly different.  Where the FN rounds have a heavy powdered bismuth head encased in a hard plastic shell the FS rounds feature a much lighter plastic shell that encases a small paint payload.  The tail section which contains either paint or PAVA on the FN rounds is left empty on the FS rounds and overall they are much lighter.  They don’t seem to load quite as smoothly as FN rounds which I attribute to their softer construction however they feed reliably.  I don’t have a chronograph but it’s quite obvious that the FS rounds have a much higher velocity and they seem to nearly vaporize the paint “warhead” rather than leave a mark.  When fired against a wood fence they leave as deep a crater as the FN rounds and break down into very small plastic pieces.

The RAP4 rounds were suggested by a poster on PbNation and seemed like an awesome option as they are inexpensive at 12 cents per projectile.  I’m not sure what the relationship is between First Strike and RAP4 is however as best as I can determine they are a solid plastic variant that is intended for testing.  The entire projectile is molded plastic, which seems similar to Zytel, with a solid head and empty tail section.  Like the FS rounds the RAP4 projectiles are much lighter than FN and fire at a much higher velocity.  One idiosyncrasy I immediately noticed is that maybe a quarter of the rounds have a deformed tail section, this doesn’t seem to impact gross functionality however I suspect it will impact accuracy.  When fired against a wood fence the RAP4 leave a impressively deep crater however stay completely intact and reusable.  Loading and feeding is as easy and reliable as the FN rounds which seems to make sense as they are much more rigid than the FS rounds.  These rounds are also reusable.  I have several projectiles which have been fired half a dozen times at various hard-ish objects and remain undamaged.

From my reading on the FN303 it seemed that traditional paintballs were rather unsuitable for the FN303 as they would be either eaten by the heavily sprung magazine or shredded by the overly powerful blast of air. While I was searching for the RAP4 rounds I saw these “rubber” training / crowd control balls and figured they were worth trying as it didn’t add to the shipping cost.  They were a bit squishier than I expected which proved completely unusable as they just kind of stay squished in the magazine and the launcher just makes a sad sort of sigh as the bolt squishes into the ball and the high pressure air just leaks out.  After a lot of fiddling about I was able to finally able to get a single rubber ball to fire by holding my finger against the rear rotating portion of the magazine so that there was zero pressure from the follower.  The ball seemed to exit at extremely high velocity however it went off at a bit of an angle I was unable to determine where it hit.  Given the hassle of just getting it to fire and the apparent gross inaccuracy I did not bother with them any further.

Though I have shot a fair number of FN rounds with quite satisfactory results since acquiring my FN303s I was curious to see how they all stacked up. A few weeks ago I was able to score a one shot kill against the squirrel mafia which seems intent on raiding my bird feeders but blowing a $4 PAVA round to whack a rat with a fancy tail didn’t seem worth it.  The FS and RAP4 rounds were obviously firing at a much higher velocity than the FN rounds due to their much lower mass but they also seemed to be a bit more random in their point of impact.  To give a rough test of accuracy I set up targets at 20 yards and gave it a shot.

For those that want to get all technical I shot the rounds with 1500 psi left in the tank and shot three FN rounds followed by for FS rounds then four RAP4 rounds.  I didn’t bother to clean the barrel in between or anything like that and the pressure was down to about 1100 psi when I was done, that did include a bunch of messing around trying to get the rubber balls to fire.  The FN303 may be the worlds most kick ass paintball gun but compared to a regular firearm it is not nearly as accurate.  The “lower” which is all the operating bits of the launcher is a rather loose fit to the “upper” which is the buttstock and mounting rails.  This loose fit means that even with a zeroed optic the point of impact at 30 yards can be skewed a couple inches either way depending on how the launcher is being held.  For the sake of consistency I braced the launcher for this test which skewed the point of impact to the left.

The largest spread between the FN rounds was approximately  three inches and made a pleasing splash of yellow paint on the target.  I bring them  first as they seem to be the accuracy standards against which the others can be judged.  The FS rounds made quite impressive paper shredding impact craters however the paint marks were marginal at best.  The group size was double the size of FN at approximately six inches.  Finally the RAP4 rounds delivered an impressive thwack to the wooden backstop however the spread was again double that of the FN rounds.  This was with rounds which I had selected for concentricy, at some later point I will test how rounds with deformed tails perform.

FN yellow permanent paint
First Strike



In conclusion the FN rounds are notably more consistent however the FS and RAP4 are more than adequate for testing and low cost plinking.  It’s worth emphasizing that the FN303, although based on the Automag design, is not a paintball gun for recreational use.  The velocity of FS rounds is much higher than they were designed for, which may have contributed to the lower accuracy, and could cause serious injury.

I finally found my scale in order to weigh the various compatible rounds.  It turns out that the Rap4 and FS rounds are roughly 1/3 the weight of the FN rounds.





















PbNation Mil Sim & Mag Fed forum:


First Strike:


RAP4 projectiles:

Shaped Projectile (Solid) (500ct) (Red)

Modified FN303 for paintball:


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