Help needed with Bren mk II

Ok, this is my first post on these forums so I will try to keep this short. I am buying a Bren mk II and need advice, the receiver is a reweld and converted to semi auto, the welding looks pretty good, but the seller said he has had a problem when firing it. He stated he used cheap ammo and on some occasions the bolt has not gone all the way back to the sear and as a result kept firing, he is not sure if it is a problem with the bolt or something else? I am hoping someone can give me a few ideas of what could be causing the problem. I have already paid for the rifle but told him not to ship it until I know what might be wrong.
I thank you in advance for your suggestions.

THINK SIMPLE. There is only ONE thing that drives the piston/breech block rearwards and that's GAS - and plenty of it. If it ain't going back far enough then there's only TWO things stopping it. Not enough gas or excess friction in the body or return spring(s)

I'd start with some known ammo, such as ex UK military RG or RL or K and if it doesn't return the piston, piston extension/breech block assembly (we call them 'the working parts') try turning up the gas.

But as we're not familiar with your semi only gun, I can't comment further

I thought Semi-Auto's in the US. Were made to a design that fired from a CLOSED bolt, & were thus striker fired? Dimentionaly different internaly to prevent Full Auto Componants from being inserted & used?

As Peter Has stated. If ANY of the working parts are not recoiling sufficiently to ensure they going rearwards. To catch on the sear, It is ALMOST certainly lack of Gas. Turn your gas regulator round to the Next Larger port.

With the gun Field stripped, Hold the upper reciever up horizontaly, & your right hand over the reciever opening that the gas piston & Breechblock are inserted from.
raise your left arm slowly holding the right one horizontal. keep raising until the piston group move to the right/rearwards. They should move smoothly. if they dont, observe where they MAY be sticking & check & dress metal from that area IF, Required.

But, I would have thought they guy who did the reweld & remanufacture of this type of weapon. Would have had all this done before it was completed & put up for sale?.......
Just Points to check..............

Thanks for your replies guys, I have not seen the rifle except the pictures of it. It sounds to me like it is an open bolt and that's what I am worried about. All he said to me was that the bolt is not going all the way back to the sear on a few occasions and as a result is firing more than 1 round. If only this was an sa80, I know them like the back of my hand. Now if it is an open bolt what are my solutions to this problem

How come you know all about SA80/L85?
You've had TWO Armourers give you the same answer. Already we know that the working parts are being pushed far enough to the rear to feed and fire the next round in the magazine but just not quite far enough for the piston post to engage the sear. So we KNOW that the gas is working but there's not enough to give the working parts sufficient momentum to engage the sear. MORE GAS

In real life, with our service Brens, the rear of the piston extension will pretty well always beat against the internal buffer in the butt slide.

To be honest, you are working blind and so are we.................. Get the gun and you'll have it in front of you ready for when we ask the next questions. No gun, no concise answers

Who rewelded the rifle? The bolt carrier should drop right in with no friction what so ever. A common issue is binding. Their are two versions of the semi in use, hammer and striker, striker being the more common. It is not uncommon for the striker to not fit properly. You can eliminate the ammo question by using the new made Privi for test firing. Other than the Greek surplus that has all but dried up, it is the only reliable ammo. That is why most of the Bren guys have a backup 7.62x54r conversion. Cheep reliable ammo, readily available. If you are truley worried about the gun call up SMG and have it sent directly to them. They will work out the bugs and then send it to you. The are really good at what they do.

Thinking out aloud C130pilot, It can't be distortion because it's already going pretty well all the way rearwards AND it's going forwards too

you said you have seen pics
show us the pics and then you might get the answer you need
is it a home reweld or a manufacturer reweld?

From the look of the weld it is a home weld, I have seen better but I have also seen a lot worse. I will upload pictures as soon as I get home from work. I am currently looking for another kit as I would like to try to build my own, but I also like the work project guns do

Unanswered question Pengman......... How come you know about our SA80 rifles? I understand that there are only very few in the US, including one of each L86/L86 plus a parts set to convert to A2 spec for each - that we gave to the US Military a few years ago