ammuntion, headspace, primers, and accuracy

[armouredbear]

after a headspace thread got shut down on another forum, i thought i'd post my question here about this so as not to keep bringing up a sensitive topic there. i have just started shooting my 1916 LSA N°1 Mk III and have had some primers backing out. they have been backing out of winchester and remington brass, both in remington factory loads (180gr), and handloads (sierra matchking 172gr HPBT). the numbers have fallen as we've shot the brass more. that said, accuracy has been strong - and i'm shooting open sights.

at 25 yards:Attachment 18534Attachment 18531

at 50 yards:Attachment 18535

at 100 yards:Attachment 18533

(she shoots better than i do)

we also tried 150gr sierra spitzer handloads at 50 yards (not nearly as accurate):
Attachment 18532

Alan de Enfield mentioned that the primers are probably backing out in all of the brass, but are being pushed back in during the recoil cycle. i'm not sure i understand why, after looking at the "headspace 101 for .303" that it would not back the brass back onto the primer every time?

now, today, because i have to go back to brooklyn at the end of the week and will not be taking my LSA back, i (perhaps recklessly, since i haven't been able to check the headspace) took her out to the range again and brought some PRVI partisan factory 172gr ammo. the rounds gave significantly more resistance while opening and closing the bolt. the first 10 rounds i fired were PRVI. they were hitting as much as 7" below POA at 50 yards. we switched back to the sierra HPBT handloads and they were also shooting dramatically low. we then tried the spitzers, which followed suit as well. accuracy seemed to have gone all to hell. the sights had not been mishandled in any way and the rifle had been cleaned and oiled after the previous day at the range. i had removed the bolt head from the bolt during cleaning to measure it and check it. the only other significant difference was that the previous time out it was about 35°F/2°C and clear, and this time is was around 55°F/13°C and drizzling. wind was nominal.

50 yards:Attachment 18538
POA was the black 1" square

100 yards:Attachment 18537
POA was the black 1"square

200 yards:Attachment 18536
POA was the target center (the other group here is from my dad's 03a3, to make sure that it wasn't just my terrible shooting)

interestingly, none of the PRVI primers were backed out, though most of them looked as if they had lost most of their bevel (opposite of the inward bevel of the primer pocket). they also stretched noticeably less, but this was expected since the brass is thicker:

Attachment 18541Attachment 18540Attachment 18539


what horrible thing have i done to my rifle? why would accuracy suddenly drop off so sharply like this? forgive me if i am being alarmist, but this is my first rifle and though my father is well versed with firearms, he has no experience with enfields - and you guys do

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[Thunderbox]

Backing out primers are a typical sign of the handload being too weak. There should be no resistance when chambering and ejecting decent ammo such as Privi - are you sure the bolthead on your rifle is fully screwed into the bolt body?

You probably just need to shoot a few more groups, in order to deduce a pattern between the ammo types you are using.

Some No1s are highly sensitive to variations in ammunition and bullet shape, and/or the bedding system (the interaction of front trigger guard screw, inner band, front spring stud and nosecap). An average No1 group should be roughly circular in shape, bigger or smaller depending upon the accuracy of the rifle/ammo combination. Some of your groups display vertical stringing and/or split groups. This would indicate to me that the bedding needs to be checked - that the screws are firm, and that nothing is missing from inside the forend, and that the forend is a good fit.

I'd personally use one batch of decent ammo - probably the Privi - and then work through this sequence:

1. check that the forend is undamaged, particularly in the recoil faces. Repair if necessary.

2. Make sure that the inner band spring is present, and also the front spring and stud;

3. assemble the rifle, ensuring the front trigger guard screw if firm. Tighten the inner band screw, then slacken it off so that the spring is just under compression - about 1 turn;

4. fire a sequence of five-shot groups, tightening the inner band screw by about 1/8 turn - observe to see if the rifle reacts significantly (I've seen some rifles show a 3-4 moa group difference on just a fraction of a turn of the inner band screw);

5. if (4) does not produce an effect, try fitting a different nosecap.

6. If you can find any, try a few test groups with surplus cordite ammo - this will provide a benchmark for the rifle.

[armouredbear]

i will try this. thank you. why the variation between the ammunition? the groups seemed much tighter (and seemingly improving) prior to firing the PRVI. also, why would the primers back out on some but not all of the rounds? i'm not sure i understand the physics of this.

[browningautorifle]

I wouldn't try too hard to mind meld with this thing...there's about three or four members that used to be weapons techs on the 303 families of weapons and they can help sort the problem out.

[Bruce_in_Oz]

Regarding primers "backing out:

At ignition, ALL primers try to back out to some extent. Let's face it, they are held in sporting ammo by friction alone. Most military stuff has annular or stake crimping to restrict movement, but there will always be some degree of movement. After all, the blast from the primer is quite brisk and not all of it goes through the flash hole(s) unimpeded.

If the cartridge case can expand or slide rearwards when the main charge ignites, the case head will slip back and re-capture the primer. Revolver shooters will be familiar with this set of events, especially the fun that can ensue when shooting very light loads. The primer will back out against the recoil face of the frame and, if the powder charge is REALLY limp, the case will not back up onto the primer and then bounce forwards. The result is that, after a few rounds, the drag of the protruding primers on the frame make the revolver "seize up".

Really light rifle loads can result in primer protrusion. Normal loads and excessive headspace can do the same if the pressure is sufficient to hold the outer wall of the cartridge against the chamber until pressure drops to a level where the brass springs back just clear of the chamber wall. If you want to shoot really light loads in a rifle calibre, use a rimmed cartridge. Rimless cases get hammered forward and the shoulder thus is hammered back, every time you pull the trigger. As rimless headspace is usually set by the shoulder, rearward movement of the shoulder is not to be encouraged.

For the sake of your brass and your peace of mind, find a gunsmith or fellow shooter with a set of .303 headspace gauges. Unlike the no4 series with its incremental bolt head sizes, You may also need to find a selection of No1 bolt heads. These are all "nominally" a standard size, however, manufacturing tolerances of the day meant that there is quite a variety of lengths out there. If you are REALLY lucky, someone out there may have a few of the post-WW2 Australian "oversize" bolt heads. Ditto for bolt bodies, although fitting a replacement bolt is not something to be undertaken lightly on something nearly 100 years old. Some of this stuff is starting to show a little wear lately.

Finally, check your cases. I have noted the occasional batch of brass with thinner than spec rims. This is a royal pain. In the glory days of .303 full-bore range shooting with "issued" ammo, it was common to see the more hard-core types run a Sweets rim-thickness gauge over their ammo before a match. The idea was that if all the ammo had the same rim thickness, the the variations in ignition timing and "case shuffle" would be reduced. The REALLY hard-core also gauged projectile diameter with a nifty little gauge plate with an indicator arm.

[armouredbear]

Thanks for the calming advice. I figure i'm just overthinking things. The primer problem, after examining our loading references some more, seems to be as a result of light loading. The headspace gauges are on their way, just to be safe, and I'm keeping an eye open for bolt heads. now I just have to wait six months before I get to shoot it again! In the meantime, we're going to change powders and get ready for summer. Now to find myself a nice aperture sight... Thanks again.