Go to Browning Firearms website, and ask about replacement parts. Or, go to a local gunshow, and ask around the dealers at the show.
Another option is replacing both pieces with a synthetic stock-forearm set from Ramline or others. They are very durable and will not crack, chip or peel ever. They also look terrible compared to wood. This is the cheapest option by far, though, because you'll never have an issue again.
Not being sure of your starting point, reassembly of an A5 can be difficult if the integral parts have been broken down. If your question is directed at re-attaching just the barrel and forearm, that is much simpler. There are two links below that should help you. One is the A5 Owners manual that you can download for free from Browning. This will show you the simple takedown of the barrel and forearm as well as reassembly. Check this source first. The second link is a source for the Auto 5 Gunsmithing video which is recommended if you have need to reassemble the receiver, integral parts, carrier or butt stock. If you do not have some gunsmithing experience, taking on an A5 will be more advanced gunsmithing and may need professional assistance.
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