Barrel heat causing pressure signs

[adam_p]

Anybody seen this before?

About 8th to 10th round gets tight and shows an ejector mark. This has happened multiple times now. I've shot 2 different factory loads after the gun gets hot and my handloads start showing pressure signs, known good shooting factory loads and they both had a sticky bolt and a little ejector mark. Let the gun cool and it is back to normal bolt lift and no ejector marks.

These are all shot with a suppressor so I'm sure it is getting hotter than if it wasn't suppressed.

[duckhunter175]

Cooling period between groups?? Gun AND ammo both out of the sun?? If the prop temp is increased that poweder will burn noticably quicker and I guess could pressure up quickly.

Also-- if you are firing and then chamber the next round but it sits in a HOT chamber for awhile it will for sure heat that round up increasing pressure and velocity. Did you notice the pressured round hitting high at all??

[adam_p]

Cooling probably 15-20 minutes between strings. Long enough for the suppressor to be cooled off enough to touch comfortably. The first string does it too. I was in the shade today. No rounds sitting in the chamber for longer than it takes to get settled and shoot. Last 2 groups today were .457” and .445” today. The .457 group had 4 rounds in 1 hole and one off to the side a little, the off to the side round was not the pressure round. No velocity jumps either on the pressure rounds. It’s weird, I even bought a magnetospeed because I didn’t trust my other chronograph when it was showing the same thing last time.

[RifleBowPistol]

Have you tried it with the suppressor off, to see what it does?

What is the rifle, what caliber, then what barrel and receiver material?

How hot is the chamber end of the barrel when the bolt starts sticking?

How many rounds are you shooting in a row, and how much time between those rounds. You said the 8 th. to 10 th. shots, the bolt gets tight?

[adam_p]

I have not shot it without the suppressor. It’s a blueprinted 700 with a 16” proof barrel in 6.5 CM. That’s 8 to 10 shots in a string, just enough time to get back on the target. Not rapid firing, just deliberate aimed shots. It’s a hunting rifle that will probably never get more than 5 shots off at a time so I’m not overly concerned about it but I’d like to know why it’s doing it.

[adam_p]

[adam_p]

Both are 5 shot groups

[RifleBowPistol]

So Proof carbon fiber barrel?
Pull the suppressor and shoot the rifle the same way. then see what happens. Then also check to see how hot the chamber end of the barrel is getting after starting around 7 shots. I would bet a carbon fiber barrel does not transfer heat out of the metal of the barrel very quickly, that's likely one possible problem. The carbon fiber, probably does not get very hot, ever, since it is a carbon fiber, epoxy combo. The epoxy is not going to transfer heat much at all. If that is the case, the metal that makes up the barrel, is going to get hot faster and not cool down all that quickly, because most of the metal that makes up the barrel will not be in contact with air. So there will be very little air cooling. Getting one of those barrel cooling fans and blowing air down the barrel would do a lot for cooling the barrel. The more metal mass a part is made of, the longer it will take to cool, then the more surface area that piece has, the better the air cooling will be. With a short carbon fiber barrel, there will be very little metal that will be in contact with air, so air cooling will be very poor. This would be my guess on potentially what you are dealing with.

The other thing I wonder about, is if somehow the suppressor is causing the pressure to increase after it gets hot. There are multiple possible ideas on how the suppressor could be increasing pressures, as it gets hot. One of those ideas is back to the barrel being carbon fiber wrapped, and not air cooling. The mass of the barrel is going to help cool the suppressor. Since the two are directly attached to each other, whichever one heats up faster, will transfer heat into the other. If you have a barrel with a lot of mass and surface area, if the suppressor were to heat up quicker than the barrel, that heat will be transferred into the barrel. All of the components attached directly together are going to try and equalize temperature. Basically if one part attached to some other part heats up quicker, that the piece it's attached to, it will transfer heat into the other part, which will lower the temp of the part heating up quicker. Typically the smaller thinner metal parts will heat up quicker. But there is a lot of heat created in the barrel from the friction of the bullet going down the barrel. I have found that because of the bullets gaining speed as they travel down the barrel and typically the muzzle end of the barrel having much less mass than the receiver end of the barrel. The muzzle end of a barrel heats up much quicker. Then add to that the burning powder that escapes the end of the muzzle, at some very high temps. With no suppressor, those gasses are probably not going to add a whole lot of temp to the barrel. But with a suppressor on the end of the barrel, depending on the design of the suppressor and the material it's made of. I can see a suppressor getting heated up a lot by those burning, and expanding gasses, coming out the end of the muzzle. It's very possible the suppressor is going to heat up faster than a barrel normally would. Then that suppressor being attached to a barrel with very little metal mass and very little surface area for air cooling. That suppressor could cause the metal of the barrel to heat up much faster than it normally would.

It would be interesting to know how much that suppressor is affecting this problem you are having. Then getting your hands on a infrared thermometer, then measure barrel temps on the chamber end, muzzle end, and then the suppressor temp. Do this before you fire the first shot, then measure temps at all three places and log those temps, all the way through to 10 rounds. Then also measure the surface temp of the carbon fiber to see if my theory is correct on the carbon fiber not transferring heat. So four measurements 11 times. Then probably take those same four measurements every 2 to 4 minutes after the last shot was fired, for about 15 minutes after the last shot was fired. It would be interesting to see how that combo heats up, as you send rounds through it.