Heavy Bolt With Mild Load

[Mlutz60]

I started load development for a 6 Creedmoor and have run into heavy bolt lift with a very light load. I have read several other threads on this and haven't found what could be causing my issue.

Component list:
Tikka action
Carbon 6 pre fit (it was chambered short when I got it so instead of sending it back I had my local smith correct the headspace)
New Alpha brass LRP( New headspace measured 1.575)
Berger 105 VLD Hunters
Federal 210
H4350

For the first outing I loaded 37.5g-42.2g in .4 increments seated .030 off the lands. Ran into slight heavy bolt lift at 40.6g and at 41g the first shot did not have heavy lift but the next 2 did so I stopped for the day. When I got home I started measuring cases to see how much they grew and what I found was the headspace on most of the first rounds fired in a group didn't grow but the second shot grew .002 the third shot grew .003 and a couple grew .004. I then started to chamber fired cases to see at what point I would feel resistance when closing the bolt and this happened when headspace was 1.578.

I figured I found where I was seeing pressure so I went down a bit and started at 36.6g up to 40.4 in .4g increments. Weather was about 15 degrees warmer this time but I then got heavy bolt lift at 38.6g and since I was 2g lighter than when I saw pressure the last time I decided to try the next set at 39g and there was no issue but at 39.4g I had heavy bolt lift again. At home when I measured cases that had heavy bolt lift they measured .001-.003 longer than cases without heavy lift.

On cases with heavy lift a bullet will drop into the case so I don't think the necks are to tight. Before the first outing I ran some oil and then dry patches through the barrel just to make sure any metal chips were removed and then cleaned it before the second outing making sure I mopped the chamber. I ran a bore scope in the chamber and didn't see anything out of the ordinary.

Just incase there is something I am missing I am including pictures of the fired brass and the bolt head. There are slight swipe marks on case that had heavy lift.

 

[Shoots 700's]

Alpha has shown pressure early in some calibers. Try Lapua and see what happens

 

[Rust]

First thing, is you load filling a minimum of 85% of the case. Low case fill can rarely cause weird pressure excursions depending on how the powder is laying in the case. P.O. Ackely chased that problem some years back and IIRC there was some more recent experimentation along the same lines. Otherwise...

Weigh the heavy lift cases in comparison to the others, compare the diameters above the web, check case length (a too long case where the neck can pinch the bullet will definitely cause problems), check case capacity. Section the heavy lift cases and one of the others to see if there are any differences.

Maybe use Lapua? I got to the point where I wouldn't build a rifle unless I bought a large quantity of Lapua for that chambering first.

 

[Yondering]

I've run into heavy bolt lift in several rifles that had nothing to do with high pressure, but they would do it with some kinds of brass but not others. The issue was a rough chamber.

You said your gunsmith had to chamber it - not knowing your gunsmith or his chambering process, it's quite possible this is the issue. If it is, you'll see roughness and/or a lot of small bright rings around the body of the fired brass. If you'd like to post a nice clear picture of the case bodies from a few pieces that had heavy bolt lift, I could point out what I'm describing if it's there.

Honing and polishing the chamber fixes it.

 

[bax]

I want to talk about the cases. Call the bottom row A, next row up B, third from the bottom C, and so forth. First case on the left is 1, center is 2, right case is 3. I see ejector swipe on A1 and A2, B1, C3, maybe on C2. Ejector swipe is caused by case brass being forced into the ejector hold and this excess brass being sheared off when opening the bolt. Pressure causes the brass to flow - either high pressure or soft brass. If those swipes match the heavy lift, you are getting somewhere. So probably pressure. Is there a (lighter) load where none of the cases swipe? Is it just a few cases that are not strong enough? Do all of the cases swipe at moderate pressure? Lots of ways to go from here:

• Try the same load with different brass.
• Pick two cases, one that swiped before and one that didn't, load both to the point where you saw swipe before and see if the pattern repeats.
• Pick a case that swiped before, load it lighter and see if it swipes.

Thinking about the comment above about Alpha brass. Could be that this whole lot is not as hard as it should be or perhaps it is just some of the cases. Maybe use it for lighter loads? Toss it all?

 

[todde]

Check the measurement at the .200 mark on fired cases with heavy bolt lift.
Todd

 

[rustyinbend]

Maybe I missed it, but ... is this virgin brass, or has it been fired/resized? Virgin brass will sometimes do this, when fired/sized brass settles down. Just a thought.

 

[Doom]

It's interesting that the barrel was chambered short (how was this determined?) and the gunsmith reamed the headspace as opposed to adjusting the the barrel. Something doesn't sound right here. Did the gunsmith install the barrel or did you? You probably have a combination of reamers used and without knowing what the freebore dimension on them you really don't know what chamber you have. Do you know the distance to the lands for your loads?

You gave the datum dimension as 1.575" on the new brass, what is the dimension on the fired brass?

 

[4O6shootist]

Alpha ocd brass is very hard at the lower half of case and web area, it doesn't expand to grip the chamber very well on virgin firing or if your dies size a lot at lower side, like a Wilson or matchmaster. A slightly oversized chamber magnifies this issue. Had the same problem in my Tikka 65cm. Switched to lapua and psi signs disappeared until 110fps higher in charge weight.