K31 bolt sticky with reloaded rounds

[jeanlikethis]

I found my K31, normally a very smooth bolt, is sticky when shooting 41gr 4064 (168 HPBT) rounds.

You need to pull quite hard to have the bolt open after firing above said rounds. 39 gr 4064 charge has a little bit less sticky then 41 gr charge.

I was using GP11 fire formed case for reloading. I checked the case and primer, no sigh of over pressure. But very hard to open the bolt, my feeling is that the case is adhered to the chamber some how and you need to pull very hard to get it out.

Then I tried GP11 surplus rounds and it is ok, bolt opens smoothly as normal.

Is the GP11 case small volumed so 41 gr of 4064 causes the over pressure ?

 

[Eaglelord17]

According to my reloading book a 168grn bullet with 4046 is listed at 40.8 for 2500ft/sec, well 43.2grns is max load at 2600ft/sec. Did you full length size the case?

I also took case volume measurements of a GP-11 case and a PPU case and the GP-11 had 67.8 well the PPU had 67.5, so they are very close to the same.

 

[Keithjohn]

The gp11 bullets are quite tapered . I made my reloads with Berger bullets the same length as gp11 and felt slight resistance closing ( didn't fire them ) and found I needed to seat the bullets deeper ( they were touching the lands ) so your OAL may be to long causing over pressure

 

[Keithjohn]

I checked ammo with my Forster comparator to find above solution

 

[kawicrash]

Remember, you don't get very much primary extraction advantage with a straight pull action. Is the case length OK? Also be very careful with COAL in a K31. Diopter has a list of COAL with various bullets for the K31.
Are you using the Tul Ammo Berdan primers?

 

[jeanlikethis]

I checked my reloading total length by trying out on my k31 so closing the bolt feel no resistance.

I am using the Berdan Dominion Primer, the ones designed for 7.62x39, a little bit weak for 7.5x55, but could ignite the 4064 though.

 

[jeanlikethis]

Remember, you don't get very much primary extraction advantage with a straight pull action. Is the case length OK?

All GP11 case has been trimmed before reloading and I didnot measure the case length after firing yet.

One more thing to note is that I did replace some of the GP11 bullets with Hornady 168gr SST bullets. I sit the bullet more deeper. Those bullets didn't product the sticky bolt as I remember.

 

[Shooting Centre]

Remember, you don't get very much primary extraction advantage with a straight pull action. Is the case length OK? Also be very careful with COAL in a K31. Diopter has a list of COAL with various bullets for the K31.
Are you using the Tul Ammo Berdan primers?

This. Straight pull rifles usually required full length sizing because of the lack of primary extraction. You said cases were fire formed. This implies you might have only neck sized.

 

[tacfoley]

The generally accepted wisdom among K31 shooters is that they really do need full-length resizing after every firing. I've been shooting mine since 1989 with one-time use GP11, but having run out of GP11 a few years back, very quickly learnt that this was the way to go to ensure easy [re]loading.

Reloading? What's the rush?

tac

 

[jeanlikethis]

Yes, I just neck size. I also read some one mentioned "partially full sizing" the case.

 

[kawicrash]

Standard 7.5x55 full-length sizing die dimensions are for the K11 and 96/11 chamber that is tighter than the K31 chamber. Hornady and others make a K31 specific full-length set, but I just use the standard Lee set with no problems. I don't screw it all the way down, I guess you could consider this partial full-length sizing.
I also have a Lee collet set, but I haven't tried them yet. These size only the neck, so I'll see what happens when I try them.

 

[Hitzy]

I run 150gr Hornady fmj at just under 2700fps with 46gr Varget, I'm up to 5 neck sizings with no issues so far with sticky bolt, it's mild and it's a stupid accurate load so I see no need to speed it up any more.
Using Lee collet dies.