How much can fps change from barrel length?

[Tubs]

How much fps difference should i expect from my reloading manual for my barrel,The manual is testing a 26" barrel and mine is only 20". I'm trying to find the best grouping for my gun but my fps is proving lower than the book states for the powder amount. E.g. 22.1grn of N130 book states 3200 fps, but the chrony shows from 3014- 3046 fps. The gun is 223 and using 50grn V max bullets all brass was FL sized and the same make, i understand there could be a small tolerance from weighing on beam scales on my behalf. I'm only .8grns off max powder load from the book which would be 3300fps would .8 grns bring it up 254fps.

 

[gbal]

How much fps difference should i expect from my reloading manual for my barrel,The manual is testing a 26" barrel and mine is only 20". I'm trying to find the best grouping for my gun but my fps is proving lower than the book states for the powder amount. E.g. 22.1grn of N130 book states 3200 fps, but the chrony shows from 3014- 3046 fps. The gun is 223 and using 50grn V max bullets all brass was FL sized and the same make, i understand there could be a small tolerance from weighing on beam scales on my behalf. I'm only .8grns off max powder load from the book which would be 3300fps would .8 grns bring it up 254fps.

 

th

 

Tubs,this similar point comes up often-I checked some data fro a Bruce Potts barrel reduvction in 223,to get a ball park figure (it varies with the MV/calibre-and just what the starting/finishing length is.Bruce lost about 35 fps i his 223 going from 24 to 18....so lets take that as ball park for 223 (individual rifles vary,whethe chopped or not,and manuals can be no more than a reasonable guide.

But seems about right- you are 6 " short of the 'manual' length,so might expect to lose somehing like (6x30fps,as you are not going so short...which results in (3200-6x30) which is 3020,within your chrono spread...which is incidentally quite wide at 30+ fps for a few shots.So nothing untoward. If you look at the book increases per increse in powder,you will get some idea of what you might expect,though a bit less because of the shorter barrel- you won't get the full 3300,with the 'book' load-because you have a shorter barrel.....you just have to accept your MV will always be down arround 180 fps or so.....but so what? What do you think 3200fps can do that 3020 fps can't? Think of an inch more drop at 300y-ball park,easy click up on scope-though you do have to fiels test.

It would help consistency (group size) if you got an improved SD for your Mvs ( +/-15 fps would be better).BUt it variation that destroys accuracy,not reduced velocity,at least in this range. Anyhow,not much you can do-barrel will always be slower because it's shorter.

gbal

[henryo]

26" to 20" will lose you ~150-200fps in a 223.

 

Also 1gr of powder is normally ~100fps in a 223, maybe a bit more speed difference with light bullets.

 

Personally I'd be looking to hit a theoretical accuracy node at 3200 - 3250 fps, which should be safe even with the shorter barrel (with lapua brass + thick cup primers), but will probably take you over factory 'max'.

[Tubs]

[Tubs]

Sorry that was meant to say,Thanks for your replies ,I'm trying to copy a factory Norma round which shoots well in my gun and that puts the bullets out at 3293-3340 with 50grn V max, but i'm now aware i', not going to get these speeds with my powder set up so i will stay just below the max load to keep it safe ,I might even go back to a lower charge which was grouping .526"-.696" at 100m as gbal stated just adjust scope and field test more,My last batch of 30 bullets varied 3 thou over the ogive measurement can i improve on this much more.

[gbal]

Tubs, within a 100/150 fps-maybe more- except for very special reasons,better to go for max accuracy than velocity....though there is usually an accuracy node within say .5 g of manual maximum. Bullets of course vary in length,from the box,so use the ogive /comparator for reference point-it is usually far more consistent,and powder charge within .1 g,if possible.It will however hardly matter at 100y,and by 300y, wind etc will be a far bigger source of dispersion. Rifles too have intrinsic limits to their precision,as well as preferences esp in bullets. Good shooting.

gbal