A reciprocating saw WILL do the job. If that is what you have and you do not wish to buy/borrow/rent a chainsaw, then you can use it. Get a long blade, designed for cutting wood. These blades have large, sharp teeth with wide gullets between them.
The reciprocating saw will take longer to do the job, and it does have some vibration issues, but I can assert that you can use it for hours at a time. A good saw is a beast, designed for heavy duty work. (I did so to cut up the wood after we had an ice storm once. It left piles of wood behind that can only be measured as many cubic yards. My chainsaw was dead, and was painfully unwilling to be revived. Amazingly, I used only 3 blades for the entire job. The final pile of wood and brush was roughly 10 feet wide, 6 feet high, and 60 feet long.)
If you do use a reciprocating saw for this purpose, you might buy some gloves with padding in the palms. Good gloves will be a necessity anyway, so use them. Ear protection is a good idea too.
If you already have a reciprocating saw and a circular saw, and want to just buy the right blades ... I'd consider a carbide blade in the circular saw, using it for wood that is up to 3 inches thick. Choose a blade with few teeth and large gullets. Then use the reciprocating saw with a wood blade only for those pieces that are too thick.
Having said all that, perhaps another solution if you do not wish to buy a gas powered chainsaw, is an electric chainsaw. These saws have no issues with gasoline, so are slightly simpler to deal with. It is still a chainsaw though. You will still need to use oil, and kickbacks are still a problem to watch out for, but a circular saw can also be dangerous if used unwisely.
Replace or sharpen dull blades as appropriate. A dull blade is a dangerous blade. It takes more effort to use, so you push harder. A good saw with a sharp blade should almost glide through the wood.
Finally, if you are cutting up a large amount of wood, then be careful. Do not work when you are tired. This is true using ANY saw. Stop and finish the job another day, a far better idea than spending time in the emergency room.
What you just described (cutting multiple pieces at the same time) is in the construction field referred to as "gang cutting". And, yes, if your only power tool is a circular saw, than to get the most precise and exact lengths, cut them in a gang cut. What you should first do is make an initial gang cut at the bottom so as to square the ends. Than measure and mark the length. After they are clamped strike a line with a square across all pieces. If you have a steady hand slowly begin the cut being sure to keep the saw blade to the waste side of the line. Or clamp a guide fence down so that when the saws base plate edge rides along it the blade is at the appropriate mark (usually 3- 3 1/2 inches from inside blade edge to outside plate edge).
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" ... a spindle lock provides an extra level of safety" from the product description on amazon. If that is not the cause, I'd phone Hitachi customer support.