What does the magazine have to do with the bolt locking back? I have TTI magazine extensions on a few mags and recently installed sprinco springs into a few of my mags. Was testing out some different buffer systems in a few of my rifles and in one I was getting inconsistent lock back. This wouldn't have anything to do with the magazine correct?

Shotgun the rifle. How's the interface between the bolt catch and follower? Pic

The follower lifts the bolt catch up which holds the bolt open.

Yes OP, it has everything to do with the mag.

What you've done is created a situation where those mags aren't putting enough spring pressure on the bolt catch to stop the BCG after last round.

Quite literally....everything.

Definitely will need some extended length springs that are designed for the additional length.  Not just "high power" springs for a standard length magazine.

It could be the buffer changes as well. This is why you only change one thing at a time.

If you’re changing buffer weights/springs and the bcg isn’t cycling back far enough for the bolt catch to clear it and engage, that would cause the same issue you are describing.

Cycle the gun by hand with an empty mag. If the bolt catch engages and holds the bolt back, while it still could be outrunning the bolt catch, it helps you to know that the magazine spring is raising the catch up enough to catch the bolt.

It's almost always a bad idea to mess with a working system. If you can, put everything back to the way it was to confirm if it is working or not.

You can usually tell if it is a buffer or magazine problem by how it feels during the last round. So if the magazine is not pushing up on the bolt release, you'll usually feel the last round slam a bit harder than normal because nothing is slowing down the bolt. It'll feel exactly like if you unloaded the rifle and pulled the bolt back and let go and it's slamming forward without anything stopping it.

However if you're feeling a sort of "hump" on the last round like something tried to stop but it kept going, that's because the magazine didn't fully raise the bolt catch in time and only partially raised it.

You can also tell if after the last round, if you pull the charging handle back is it locking the bolt back or is it slamming forward even with the magazine inserted?

Usually when it's a magazine problem, the issue will be that the bolt catch isn't able to reach the follower. Sometimes in magpul pmags they'll do this with certain gen of the rifle but in your case you're using a totally different brand.

It may also be a buffer problem where it is cycling too fast for the bolt catch to raise in time. And it may be a combination where it's cycling too fast BUT it's also not able to raise it quick enough because the spring isn't as strong due to the magazine.

Best way to figure it out is to study the last round hold. If it is one magazine doing it, I wouldn't worry about it but if it is a particular brand of magazine and you have a lot of them, then perhaps it is worth investing some time. For starters I would put the magazine back to its factory configuration and go with that.

Lesson here is to change out one system at a time in your build.

The buffer systems was mostly on the other rifles. With this one in particular I changed my regular carbine spring with an h2 to a geissele super 42 also h2 I thought it was weird it was happening to begin due to this then I put back the stock spring and same thing so just wanted some insight. They're pmag gen 3's I'm using. The TTI mags are designed to work with "stock springs" so I thought I could get away with using these "upgraded" ones but doesn't seem to be the case. I'll test vs some stock mags n see what's up

Man I can't remember the last time I felt so green. Such a basic functionality. These are images but both stock and altered mags are catching

Altered below

Correct they may lock back while inserted and manually cycled but because one has less spring tension pushing upwards the bolt is moving too fast before it is able to fully engage. The solution is to restore it back to the stock one. You can try a heavier buffer or buffer spring but it may work sometimes and not other times.

So if you think about it the spring tension from the magazine is always pushing up. As the bolt cycles, the magazine is constantly pushing upwards. The reason why you're not experiencing malfunctions during feeding might be because you're shooting semi and perhaps not quick enough to run the spring. I do believe if you were a FA lower, you would be experiencing feeding issues along with the bolt lock back issue. The problem is on the last round, the timing of the follower going up and pushing the bolt catch up is critical to catching the bolt in time as it returns home. Now you may think that that making a heavier buffer or tighter spring might work here but it may cause other issues too like feeding issues because now there is a heavier mass to push backwards along with the timing of the spring going up.

If it works fine without the extended base plate, then I would just return the base plate. Maybe even asking the company themselves what sort of spring upgrade they recommend if any.

I ran the mags on several different setups and they ran flawlessly so it's definitely the buffer system as I wasn't getting lockback with stock mags. Seems my thinking that the super 42 would operate oem just with better service life was incorrect