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Probably a stupid question, I've been discussing barrel lengths with a mate as I've got a tikka t3x with a 20 inch barrel coming he says it won't reach a 1000 yards because of the barrel length, I jokingly said but when I add my mod on to the end it'll be 25 inch.
Then I thought about it does adding a mod to a short barrel then increase the barrels length allowing more powder to be burnt kind of thing.
Not seen any thing asked about that anywhere. I'm probably being daft.

No.... once the bullet has left the confines of the barrel, the gasses behind it are free to swirl around in the chambers of the moderator, so they don't continue to push the bullet..

Some of the gasses will continue to follow the bullet, but the pressure will be much reduced.

and 20" barrels are more than able to reach 1000yds, they will lose out to the 30odd " target rifle barrels but will still work

You'll gain a small, but negligible boost to velocity using a moderator (30-50fps if memory serves). Here's some data from a 223:
https://www.thetruthaboutguns.com/2012/ ... he-bullet/

I used to have a Tikka T3 like you with the 20" varmint barrel and while I could shoot out to 1,000 yards and get hits, they weren't consistent. Using a 155gr Lapua Scenar and modest roads I was achieving velocities around 2,400-2,500fps. At 1,000 yards the bullet will be transonic which doesn't help your accuracy so much.

Rifles with 28" to 30" barrels will achieve around 2,900fps using the same bullet and I believe they remain transonic at 1,000 yards.

Actually you do pick-up some additional velocity through the 'Free boost effect' however it is not going to get you to 1000 yards with any degree of consistency. Having said that I shot 1000 yards with a 24" barrel Accuracy International 7,62x51 and it was better than harassing fire with occasional MOA groups, this was with 170 grain Lockbase and a healthy load of N540

I have a 1917 SMLE 303 with at 25 inch barrel and the sights ramp up to 2000 yards and the bullet will travel in excess of 3000 yards so in short a 308 with a 20 inch barrel will definitely travel the 1000 yards but no guarantee about accuracy as too many variables (shooter, sights,wind etc)
Also on the humble 22 LR some boxes give warnings about travelling approximately 1 mile

Those mentioning heavier bullets and transonic effects are on the money. A 155gr commercial round with a reasonably sleek bullet (eg NRA GGG 7.62 Imperial, which uses the Sierra 2155) will stay above transonic to 1000 when fired from a TR 30" barrel. This matters a lot because in the transonic zone the aerodynamic drag goes through the roof - it's about three times the supersonic drag and five times the subsonic drag. The result is that small variations in muzzle velocity result in small variations in the amout of time in transonic zone if the target happens to be in the middle of that zone, and those small variations, because of the very high drag in that regime, have a disproportionate effect on the point of impact. It is really the case that if one increases the range, so that all the shots go through all the transonic zone and out the other side, the group will tighten again. In the case of a 20" barrel you may well find that your rounds are all going subsonic around the 900y mark, and it works reasonably well at 1000. Heavier bullets will carry better, but if that puts you back into the regime where the target lies in the transonic zone, the result may be (much) worse.

Thanks for replies, not really sure I understand the bullet travelling transonic, subsonic etc.
Think I've a lot more reading to do on the subject.
I did try to get the tikka with the 24 inch barrel but seems there's between a 6/9 month wait on that barrel length.
I may still order one whilst I'm using the 20 inch then do a one for one when 24 inch arrives.
Clubs I shoot in only go out to 600 yards but me and mates have land we can go out to as far as we like so it's nothing serious.

At supersonic speeds the bullet has a high ballistic coefficient and the trajectory is elliptical. Transonic speed is the speed at which the bullets sonic boom catches up and overtakes it, and it's this sonic boom which destabilises the bullet. Subsonic trajectories are like rainbows and the bullet will drop faster compared with supersonic.

At long range this is why barrel length and bullet velocity is key, because you want to keep the bullet in the supersonic speeds as much as possible.

Redrooster wrote:Thanks for replies, not really sure I understand the bullet travelling transonic, subsonic etc.
Think I've a lot more reading to do on the subject.
I did try to get the tikka with the 24 inch barrel but seems there's between a 6/9 month wait on that barrel length.
I may still order one whilst I'm using the 20 inch then do a one for one when 24 inch arrives.
Clubs I shoot in only go out to 600 yards but me and mates have land we can go out to as far as we like so it's nothing serious.

if you only go to 600 yrds then why go to the expense and hassle of a 1for1 once a longer barrel arrives ? that 4 inchs isn't going to make a great difference and especially not for plinking. at 600 the 20in will be more than good enough.