HiCapMag Posted January 2, 2007 Share Posted January 2, 2007 First, sorry but I used search and did not find what I was looking for. Debating between an 18 and 20 inch JP upper. Need for 3 gun and some longer range stuff out to 500-600 yards on occassion. Anyone know how many fps I give up with the 18? What would you recommend? Thanks to all. Quote Link to comment Share on other sites More sharing options...
D.Hayden Posted January 2, 2007 Share Posted January 2, 2007 Usually 25-30 fps per inch. Some people say 50, but I haven't seen that much swing in mine. Quote Link to comment Share on other sites More sharing options...
TRUBL Posted January 2, 2007 Share Posted January 2, 2007 you need to comparte apples to apples.....in a perfect world and in the real world if you take your existing 20" barrel and cut it down to 18" you may see as much as 50 fps difference. HOWEVER......depending on the tolerence of the bore, chamber....an 18" barrel may even show more velocity than a 20" barrel. if you are mounting iron sights on the gun and 'want' the longest sight radius possible, then go with a 20" barrel. if you are using opetics....it is a mute point....go with an 18" barrel. Quote Link to comment Share on other sites More sharing options...
StealthyBlagga Posted January 2, 2007 Share Posted January 2, 2007 I would imagine it depends on the ammo you are using... I would expect 55s to show more velocity loss than 75s, and Power Factor is obviously more dependent on velocity with the lighter bullets. Quote Link to comment Share on other sites More sharing options...
uscbigdawg Posted January 2, 2007 Share Posted January 2, 2007 You don't lose enough to matter. Rich Quote Link to comment Share on other sites More sharing options...
George Posted January 2, 2007 Share Posted January 2, 2007 +n to the other posters. In the end, only the potential for a longer iron sight radius is lost (as Tim mentioned). There really is no downside for this sport to an 18" bbl with full length gas system if you have optics topside. Quote Link to comment Share on other sites More sharing options...
cking Posted January 2, 2007 Share Posted January 2, 2007 The theory behind the 18 is that 14/16 caused too much velocity loss so that hits on targets were not effective past 200 yards because the 223 needs high velocity to be effective, It needs to tumble, yah, and fracture to be an effective round. Same problem with the twist rate. the 1/7 twist for ss109 round. Made them too stable, the just go in and out and leave nice little hole. So if you just punching paper it don't matter, if your hunting with it, it does matter. To be effective it needs to impact at 2500fps or more. Quote Link to comment Share on other sites More sharing options...
bigbrowndog Posted January 2, 2007 Share Posted January 2, 2007 (edited) I chrono'd loads thru my 24" and 18" the other day, 55gr, 68gr, 77gr. Velocity loss from the 18" was aprox. 60-70fps across the board. thats total velocity loss.......not per inch. I was kinda suprised. Trapr Edited January 2, 2007 by bigbrowndog Quote Link to comment Share on other sites More sharing options...
George Posted January 2, 2007 Share Posted January 2, 2007 Trapr, I am not surprised and will put money on the probability that the rounds were slowing down for the last couple inches in the bbl at 24". Most .223 loads are finished burning right around the 18-20" point and any further bbl length just allows coast down as the pressure spike wanes. The trick to getting the full potential of a longer than 20" bbl is to use real slow powder in huge amounts so it is still pushing at the muzzle ;-). AA2520 and Varget/RL15 qualify, AA2230 and a lot of others do not. AR's have trouble getting as much powder in as needed because of mag length limits. Bolt rifles do not so they are easier to get the full V out of. Quote Link to comment Share on other sites More sharing options...
oddjob Posted January 2, 2007 Share Posted January 2, 2007 Mine was cut from 20" to 18". The local smith (very knowledgable too) said it won't make much of a difference in terms of accuracy & velocity for 3 gun matches. If I was shooting high power I would not have done it. Anyways I'm happy with how the gun handles and shoots. Quote Link to comment Share on other sites More sharing options...
PacMan Posted January 2, 2007 Share Posted January 2, 2007 I was pleasantly surprised to find how much easier the 18" swings than 20", didn't think 2" of barrel could make such noticeable perception Quote Link to comment Share on other sites More sharing options...
Bear1142 Posted January 3, 2007 Share Posted January 3, 2007 Go with the 18". By the time you put on a good compensator, you're looking at another 3" of muzzle length. A 20" barrel and comp. makes for a pretty long combination to manuever around in tight quarters. With the right powder selection, you'll lose very little velocity over a 20" barrel. I'm using a couple of medium/fast powders and I can get the same velocities as I would in a 20" barrel. Erik Quote Link to comment Share on other sites More sharing options...
Nick Weidhaas Posted January 3, 2007 Share Posted January 3, 2007 Just had this conversation with John at JP. According to John there is not a significant velocity loss with an 18" v. 20" bbl. John said the velocity loss is much greater when you go from a 20" to a 16" bbl. I've wanted to shoot the ITRC and John said his 18" bbl. will handle anything at this match (carbine targets out to 450 or so). I shoot a 20" JP 1:8 upper. Love the way the gun settles and it is deadly accurate. That said, I struggle a little on the faster field courses as the 20" upper is slower to index. The bbl. is 20" plus the comp, so the gun is big. Have shot my son's 17.5" upper built by Benny at TSS, and it will do everything my 20" upper will do, just a little faster. If I had to do it over again (I may yet) I'd go with a shorter upper. I'd actually like to try one of JP's new 16" 1:8 bbl., mid-length gas, match uppers. I think it would rock on the majority of the stuff we see. The only place I see the 20" gun as a must have is shooting irons. My .02 Nick- Quote Link to comment Share on other sites More sharing options...
uscbigdawg Posted January 3, 2007 Share Posted January 3, 2007 Nick, The JP 16" is pretty much what's going to be my next rifle when I come back from deployment. For Open and Tactical it just makes sense. You don't lose that much velocity and since we're using glass, sight radius is not the issue. The only concern that I have is down range thump on Larue's. Regular flashers even with 50gr. bullets are no problem, but as Matt B. and Taran said on Matt's show, the Larue's and flashers w/ flags (i.e. what we'll find at the SMM3G) need a little extra umph, especially if there's some wind. I run the 69gr. SMK's now, but will definitely be testing the 75's and 77's when I get back. Thank goodness we can get them in bulk. Rich Quote Link to comment Share on other sites More sharing options...
HiCapMag Posted January 14, 2007 Author Share Posted January 14, 2007 Thanks for all the responses. Quote Link to comment Share on other sites More sharing options...
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