Bullet Jump

[Macd]

Being doing some article research on bullet jump. Like many I have held the view that less jump is better. Short freebores are very popular in match grade barrels. I am changing my opinion. Any thoughts. I will post a link when I get a moment.

[Macd]

RD: At your discretion you may want to move this thread. Not sure why I started it in the Topic Bucket. Senior's moment I guess.

https://precisionrifleblog.com/2020/04/ ... ment-tips/

I have two 6.5x55 rifles. One is an original (barrel appears to be a armoury replacement) 1896 (M96) rifle in full military stock and the other is an 1896 converted to target rifle (CG63) with a heavy barrel and improved lock time. The comparator length for the same cartridge in the rifles is greater than 0.105 inch different, the CG63 being the shorter. Part I am sure is throat erosion but I am sure part is design. I load both to a jump of .025 and .020 respectively. In both cases the bullet is seated shallower than I like. Both rifles provide excellent accuracy with the M96 having the edge probably due to the fact I have it scoped and the CG63 has diopter sights. My thoughts are to do a trial with 20 rounds for each with progressively more jump until the bullet is seated to the cannelure. This will translate into a jump of 0.188 for the CG63 and a whopping 0.293 for the M96. Obviously going to have to adjust powder charges.

6.5x55 seating.jpg

[Kaiser]

The long throat of the M96 was for the military FMJ round nose bullets of 156 or 160gr. Thus, for all lighter weight bullets it does require a longer "jump" to the lands if the bullet is loaded where most .264 diameter bullet's cannelures are located. I would think that if a bullet (rifle only) is seated to this cannelure groove it would decrease pressures, and not necessarily decrease accuracy. This "short" seating creates a "free bore" effect which Weatherby designed in their Magnums to boost velocity without spiking pressures. I too load the 6.5X55 bullets further out in the case in my long throated M96 for additional cartridge powder capacity, but I try to keep close to one (1) bullet diameter length inside the case. Having said all this, I have discovered that 100HP bullets to be extremely accurate in my rifle and they are so far from the riflings it's a joke. They do not, however, shoot anywhere close to my normal 120 or 140gr loads, and I have to shoot at the bottom of the target to get them on paper (not very useable!) Best of luck with yours.

[Macd]

The bullet is a 139 grain PRVI which has almost identical dimensions as the 139 grain "skarp patron m/94 projektil m/41 prickskytte" adopted by Sweden in 1941. The barrel of the M96 rifle has only the last 3 digits of the receiver serial number which indicates it was a replacement but no way to say when. It has an excellent bore. The M96 has a sweet spot around 2600 FPS with the PRVI bullet. It does less well with the 140 Hornady. The CG63 likes it faster and the diopter sight (Elite) is calibrated for 830 M/S (2731 FPS). The rebuild to the CG63 was done in 1973. Both the 140 Hornady and 139 PRVI shoot about as well in this rifle. I have some Norma Alaska cartridges from the 1950's. They pretty much duplicate the original 156 grain military bullet having the same lead core, steel, guiding construction and have an identical length and profile. The only difference I can see is the Norma bullet has an exposed lead tip. I have not run any QuickLoad projections on deeper seatings but I expect some pressure increase. My preferred powders are HV100 and IMR7828. Both are slow powders and I am pushing case fill with both now.

[RBHarter]

To quote another , it matters when it does .

A guy would be hard pressed to get a bullet against the lands in a Weatherby or any of the Savages I've had and keep it securely held in the case . Seriously I shot repeatable 5 shots @ .900 and less with a jump pushing 3/10 inches .

[Macd]

To quote another , it matters when it does .

A guy would be hard pressed to get a bullet against the lands in a Weatherby or any of the Savages I've had and keep it securely held in the case . Seriously I shot repeatable 5 shots @ .900 and less with a jump pushing 3/10 inches .

This is exactly what I suspected and why I started rethinking the whole seating depth/jump issue. Some of the practises I have used, such as allowing the lands to do the final seating of cast bullets I am now questioning. I know that precision shooters seek minimum jump. I have burned up a lot of powder and bullets tuning loads by adjusting bullet seating but I always start as close as I can to the lands. Some bullets and chambers do not cooperate especially most milsurps but, as can be seen above, I try to get out as far as I can. The K31 Swiss is the only exception. It has a virtually no freebore and a very short leade so the challenge is get a bullet that doesn't seat so deep that it reduces case volume.

[Macd]

So I loaded up 20 rounds for the M96 using HV100 and PRVI 139 FMJBT. I started with the bullet seated to the cannelure and went out in 0.030 increments keeping case fill at 94%. Jump went from the usual .25 to between .124 and .254. QL FPS predictions were all within 50 FPS of my current 2600 FPS. Will see what happens when I get to the range.

[RBHarter]

264 WM . Loaded just off for an unfired example .

IMG_20170830_112325320.jpg

140 Win Power Points and a paper patched NOE 260-120 . I don't remember the OAL but there's not much in the case with either bullet . It's loaded to a level of hot 6.5×55 for the jackets and almost 260 with the patch just off the lands . First load out of gate 5 under a quarter . The patch went 5 into 3.5" at 400 yd . The rifle was so stinking easy I lost interest .
FN 98' , pencil barrel 26" .

The 110 Savage 06' I fought to a finish ........ Jump made zero difference , I seated to the groove and called it good . Breach seated the 150 gr SPBT was barely into the neck enough to stay in place to get it out of the chamber .

[Macd]

Range day. Wind was blustery which made getting a steady rest a challenge.

All at 50 meters. M96 rifle. 139 grain PRVI. No crimp. Norma 1x fired, trimmed and full sized brass. Hodgdon HV100 powder. Velocities over the 20 rounds went from a low of 2517 to a high of 2583 fps. The SD was 21. I feel I was pretty successful in minimizing changes in velocity by adjusting the powder charge as I seated each group of rounds deeper by 0.030". I have to admit I was very surprised to see the deepest seated group was the best. This was seated to the bullet cannelure which gives a jump of approximately 0.293. It doesn't match the sub-MOA I have got with this rifle, bullet and powder but seated 0.025 off the lands but it is pretty good.

NOTE: Seated lengths shown are using a comparator which measures to a point on the ogive.

Length /Power 2.5 /42.0 grains --- 2.530/42.2--- 2.560/42.4, --- 2.590/42.6

20200612_162331_resized.jpg

[horseman]

My rifles are all hunting rifles and get fired seldom at targets. I also doubt I could shoot better than an inch at 100yds with any of them. Not the rifles fault. I'm the low denominator. I never load closer than .20 off the lands as best I can measure with the Hornady gauge. Factory rounds are further out than that and shoot plenty good to hunt with.

Macd's target above, I'd pull them over to center with a few clicks and at 100yds, with optics, would be pleased with those.