No. 4 "Compensation" Tests

I’ve now loaded up 42.0, 43.5 and 45.0 gr N135 with the 123 gr Lapua .311 FMJ bullet to book velocities of 2700, 2800 and 2900 fps.  Hope to get back to the range on Friday.

Very interesting. I played briefly with light 125Gr .312 bullets for the No4 a while back.The results were disappointing to say the least, maybe because of the over-stabilization.

IIRC I tried IMR 3031 & H335. The results were so disappointing I actually greyed out the load in my reloading spreadsheets.

Load data:

Case - PPU neck sized

Primer - WLR

Powder - Varget, 41.0, 42.0, and 43.0 grains

Bullet - PPU 150 gr FMJBT

Temp - 79 to 82 degrees F.

I shot the three loads at 200 yds at the same elevation settings on the scope to record the mean point of impact of the group to see if this bullet will show positive compensation with these results:

Charge  MV   Mean Elev 

 (gr)    (fps)   (MOA)

 41.0   2488    +1.2

 42.0   2564    +1.9

 43.0   2616    +3.4

Note that the increasing group elevation is substantially more than the bullet trajectory at 200 yds accounts for, indicating that bullets are exiting the muzzle at the barrel is moving downward resulting in negative compensation. 

The elevation MPI of the group rose as mean velocity increased, so these loads did not show positive compensation.  However, the elevation spread of the 43.0 gr load was just 1.0 MOA with a velocity ES of 45 fps and slow bullets hit high and fast bullets hit low on the target indicating that for velocities above about 2580 fps, this load gives positive compensation. 

Target below is the 43.0 gr load, shot 4 was called high, the remaining 9 shots had an elevation spread of 1.0 MOA.  I think this is quite good given how inexpensive the PPU bullet is (compared to a 174 gr SMK). 

I will load up more of the 43.0 gr load to try at 600 yds and also load 43.5 gr to confirm positive compensation at 600 yds. 43.0 gr Varget with 150 gr bullet is listed as max on the Hodgon website, but absolutely no indications of high pressure, easy case extraction. 

I have not tried the 150 grain bullet. Is it a flat base?

Also, what is the overall length of the cartridge?

Testing continues with the No. 4 T, with varying results.  They all show “negative compensation” with 174 grain bullets at typical velocities this bullet is loaded to.   I’ve tried three different fore-end barrel bedding methods/pressures as follows:

1) center bearing at 8 inches from the front of the action body, pressure at the muzzle to lift the barrel off the bearing is 35 lbs, no muzzle bearing,

2) as above, with pressure at 12 lbs, and

3) as for 2) above plus two layers of cork under barrel at front of fore-end bearing location to lift the muzzle 0.01” from its free position.

Yesterday testing method 2) above gave mixed results at 300 yards.  It was too windy to test at 600 yds. 

I was using my standard match load:

PPU Case Neck Sized

WLR Primer

40.3 gr Varget

174 gr SMK

3.07” OAL

View thru the No. 32 Scope at the 300 yard target. This is about what the actual sight picture looks like.  Target is the US NRA Short Range SR-3 target with a 3” diameter X ring and 7” diameter 10 ring. I didn’t get the rifle sitting on the bag rest quite right, vertical graticule was pointing to the left of the center of the target. I use a 6 O’Clock hold on the black aiming mark (8 ring). 

The photo below clearly shows my frustration with vertical stringing caused by high and low velocity shots.  8 of 10 shots held a group of just under 1 minute of angle, quite pleased with that.  Those 8 shots in the X ring had a velocity extreme spread of just 10 fps.  BUT, shot # 6 (low in 9 ring) was 32 fps below the mean velocity (2407 fps) and shot # 1 was 23 fps above the mean for a total ES of 55 fps.  A 55 fps ES should not cause this much vertical stringing at 300 yards (only about  1/2 MOA).  Barrel movement/ deflection at the moment the bullet leaves the muzzle is exaggerating the velocity differences (a very distinct “negative compensation”), this has been a repeatable condition with this rifle for all three barrel bedding methods used.  

My plan is to next try the orthodox barrel bedding, relieve the center bearing and add a muzzle bearing at 5-7 lbs pressure.  This will give me an idea of how the T performed “as issued”. 

Last week, I decided to clear out the hardwood bearing that had been fitted in the fore-end at 8 inches forward of the back of the barrel (often referred to as center bedded).  This was done in the 1960's by DCRA Technical Advisor/Armorer Dave Reynolds.  Using a barrel channel scraper, I scraped the bearing down to the surface of the fore-end channel to get clearance. 

I then added two layers of cork material under the barrel at the normal No. 4 muzzle bearing location which had been cleared out by Dave to obtain the center bedded fit.  This gave me 5 lbs of barrel pressure with barrel clear of the fore-end between muzzle bearing and chamber reinforce consistent with the standard factory stocking up procedure.  

Initial tests at 200 yards looked very promising, but the rifle soon showed the same "negative compensation" which results in vertical stringing of the group from bullet muzzle velocity variations, fast bullets impacting high and slow bullets impacting low on the target.  First three shots just over 1 inch spread at 200 yards. 

I've now removed the cork shims at the muzzle and have added a cork shim under the barrel at the sling swivel band location on the fore-end to give 14 lbs of pressure (i.e. 14 lbs of muzzle weight needed to lift the barrel off the bearing).  This bearing location was used for the No. 4 7.62 conversions and was very successful in DCRA competitions at long range by fully compensating for muzzle velocity variations as I've shown here on this thread.  

The search for accuracy never stops...