HBN coated bullets , velocity drop

[micmac]

Have been HBN coating 162 a-max for use in a pair of identical tactical .284win rifles and one thing that is bugging me lately is a 30-40fps velocity drop that follows cca 200 rounds without cleaning(exept a dry VFG pellet here and there) . Once the barrel is cleaned and patched with HBN coated patch MV settles again in couple of rounds.
Also interesting when borescoping the barrel i do not see much trace of HBN near the chamber end while last 10 or so inches is quite 'white' with HBN and interestingly enough mostly on the lands not much on the groves.

Load: 50.4 RS60 (same as RL17) ,Murom primers,lapua cases,162A-max impact coated


Both rifles are as identical as they get ,same action,barrel,chamber reamer,stock,muzzlebrake etc only thing separating them is the riflescope. Barrels are cut rifled Peurala and very uniform as same ammo registers only some 4fps difference from one gun to another.

Is this kind of MV drop to be expected with coated bullets so does it realy make any sense to coat them for tactical comps ,as 80+% of shots are coldbore and there is no foulers to foul a cleaned barrel .

 

[JRS]

Due to less friction, it reduces chamber pressure and velocity.

 

[micmac]

Yes i understand the machanics i am just suprised by rather steep and sudden drop in MV.

Also if anyone is interested log book shows MV variation of cca 3.5fps per °C in temperature 12-25°C range for which we have enough chrono data with Reload Swiss RS60 (same powder is in US labeled as RL17)

 

[jr600yd]

I can't speak for HBN but your results are common when using Moly.

 

[Baddog0302]

I had similar results with HBN, not sure about the velocity drop in FPS, but I ended up adding 0.2 to 0.4 gn's of powder to get the POI back to what it was.
By the way, what method are you using to coat your bullets ?

 

[aussiepowderburner]

I can only add that I have an F/TR barrel set up for 215 hybrids and it does exactly the same thing with naked projectiles.
It is reasonably new but my observations are that at around 100-120 rounds it loses approx 40 fps?
Only logic I have is fouling issue leading to this phenomena?
I have not seen this prior?
Once cleaned and after 3 shots or so to foul and stabilise velocity it then gives expected bullet speed once again?
APB

 

[Syncrowave]

From what I've read, it lowers MV because as JRS said, the reduction in bore friction lowers pressure – but I've also read that you can ultimately get more velocity with heavier powder charges than you can get with uncoated bullets before signs of overpressure start showing up, again because of the reduced friction in the bore.

 

[micmac]

I had similar results with HBN, not sure about the velocity drop in FPS, but I ended up adding 0.2 to 0.4 gn's of powder to get the POI back to what it was.
By the way, what method are you using to coat your bullets ?

Impact coating in a rotary tumbler

 

[jo191145]

I can only add that I have an F/TR barrel set up for 215 hybrids and it does exactly the same thing with naked projectiles.
It is reasonably new but my observations are that at around 100-120 rounds it loses approx 40 fps?
Only logic I have is fouling issue leading to this phenomena?
I have not seen this prior?
Once cleaned and after 3 shots or so to foul and stabilise velocity it then gives expected bullet speed once again?
APB

s

Interesting.
Are you thinking perhaps the bullet is contstricted by fouling in the throat area and allowing a small bit of gases to escape by it further down the bore?
I don't have the answer. Just asking. It's what occurred to me after reading the OPs post and his description of his bore,,,presumably after cleaning.